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PHP是世界上最好的语言

来源：互联网 发布：中医体质软件 编辑：程序博客网 时间：2024/05/21 06:59

1.认识PHP代码标识

想在页面中编写PHP​代码非常容易，如下面代码：

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>   <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PHP是世界上最好的语言"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

就像你可以编写JavaScript脚本语言需要写在<script></script >标签之间一样，你可以在页面中编写PHP代码写在<?php?>标签之间，但注意后面的?>是可以省略的。

也可以计算算式：<?php echo 23+ 6;?>

2.字符串

一个字符串是用双引号括起来的一个词或一个句字，比如：“Hi,jimo！”。你可以用PHP语言输出把这个字符串输出，像这样：

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hi,jimo！"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

也可以用单引号括起来，就像下面代码：

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Hi,jimo！'</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

可以把两个字符用连接符连接起来，像这样:

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Hi,'</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'jimo!'</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

注意:连接符可以连接多个字符串，上面的例子就是把一个字符串拆分为二个小字符串，然后用一个连接符连起来。

在php中字符串连接符是用点（.）来表示的，这一点比较特殊，其它语言中是用加号（+）来表示的，比如：JavaScirpt、Asp、Java。

3.PHP语句结束符

是不是已经注意在每一条PHP代码行结尾处都会有一个分号；。对的，这点注意，在PHP编程中需要在每条语句的末尾加入分号；。但要注意，分号；一定在半角状态下输入。 
4.注释 
在PHP中也有注释语句：用双斜杠（//）来表示。其它语言中，Html中使用<!--注释语句-->，CSS中使用/*注释语句*/。如下面代码：

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//输出hi,jimo!</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"hi,jimo!"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

注释语句可以在PHP程序的任意位置（但注释语句一定要写在<?php ?>之间）。

4.变量名的命名规则

变量名除了需要“$”进行标识外，还有一些规则需要遵守。就像拿到驾驶证以后，还得遵守交通规则。变量名的命名规则主要有三条：

1、变量名必须以字母或下划线 “_”开头，如”$_name”,”$name” ,”$name2”等，但是”$9name”是不对的。

2、变量名只能由字母、数字、以及“_”组成，还能包含汉字。如”$_qq”,”$qq308” ,”$my_apple”，“比如"$name我等”，但是”$name*”是不对的。

3、变量名不允许包含空格。当变量名由多个单词组成，建议使用“_”进行分隔（比如 $my_apple），俗称下划线法，或者以大写字母开头比如$myApple，俗称骆驼式命名法（也称驼峰命名法）。

特别需要注意的是，在PHP中变量名是区分大小写的，如“$my_book”与“$my_Book”代表两个不同的变量。

5.变量是什么

PHP是一门松散类型的语言，不必向PHP声明变量的数据类型，PHP会自动把变量转换为自动的数据类型，一定程度降低了学习PHP的门槛。

通过“var_dump”函数，输出数据类型:

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>  <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string</span>  = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"就是就是"</span>; var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>; <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9494</span>; var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string</span>);  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

2

在变量中，由于变量占用的空间单元不一样（占的地盘大小不一样），也分成几种数据类型，就像超市商品的包装袋，有几种不同类型，不同的商品使用不同的包装袋。我们可以通过使用“memory_get_usage”获取当前PHP消耗的内存。

<code class="hljs handlebars has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="xml" style="box-sizing: border-box;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m1</span> = memory_get_usage(); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var_string</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"123"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m2</span> = memory_get_usage()-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m1</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>; <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$n</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">123</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m3</span> = memory_get_usage()-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m1</span>-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m2</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>; <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$f</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">123.00</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m4</span> = memory_get_usage()-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m1</span>-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m2</span>-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m3</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>; <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var_array</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">array</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"123"</span>);<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m5</span> = memory_get_usage()-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m1</span>-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m2</span>-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m3</span>-<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$m4</span>; <span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

1

6.变量的数据类型

在PHP中，支持8种原始类型，其中包括四种标量类型、两种复合类型和两种特殊类型。 
1.标量类型—布尔类型 
2.标量类型—整型 
3.标量类型—浮点型 
浮点型（浮点数、双精度数或实数），也就是通常说的小数，可以用小数点或者科学计数法表示。科学计数法可以使用小写的e，也可以使用大写的E。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$num_float</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.234</span>;    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//小数点  </span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$num_float</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1.2e3</span>;    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//科学计数法，小写e  </span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$num_float</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7.0E-10</span>;     <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//科学计数法，大写E  </span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

4.标量类型—字符串（1） 
字符串是由一系列字符组成，在PHP中，字符和字节一样，也就是说，一共有256种不同字符的可能性。

字符串型可以用三种方法定义：单引号形式、双引号形式和Heredoc结构形式。

4.标量类型—字符串（2） 
当你的字符串中包含引号的时候，我们怎么办？

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string1</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'甲问："寂寞长的帅吗？"'</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string2</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"乙毫不犹豫地回答：'当然帅了！'"</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string3</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'甲问:\'能告诉我有多帅吗？\''</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//单引号嵌入单引号</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string4</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"乙答道:\"帅爆了\""</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string1</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string2</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string3</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$str_string4</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

3

4.标量类型—字符串（3） 
当引号遇到美元咋办？这里的美元可不是真正的美元，而是用美元符号标识的变量。 
规则： 
当双引号中包含变量时，变量会与双引号中的内容连接在一起； 
当单引号中包含变量时，变量会被当做字符串输出。

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$love</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"I love you!"</span>; <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string1</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"寂寞，$love"</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string2</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'寂寞,$love'</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string1</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string2</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

4

4.标量类型—字符串（4） 
当我的字符串很长怎么办？

我们可以使用Heredoc结构形式的方法来解决该问题，首先使用定界符表示字符串（<<<），接着在“<<<“之后提供一个标识符GOD，然后是字符串，最后以提供的这个标识符结束字符串。

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string1</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;"><<<GOD我有一只小毛驴，我从来也不骑。有一天我心血来潮，骑着去赶集。我手里拿着小皮鞭，我心里正得意。不知怎么哗啦啦啦啦，我摔了一身泥.GOD;</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$string1</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

5

注意：在结尾的一行，一定要另起一行，并且此行除了“GOD”，并以“；”号结束之外，不能有任何其他字符，前后都不能有，包括空格，否则会出现错误的哦。切记切记哦。

5.第一种特殊类型—资源 
资源（resource）：资源是由专门的函数来建立和使用的，例如打开文件、数据连接、图形画布。我们可以对资源进行操作（创建、使用和释放）。任何资源，在不需要的时候应该被及时释放。如果我们忘记了释放资源，系统自动启用垃圾回收机制，在页面执行完毕后回收资源，以避免内存被消耗殆尽。

举例如下：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$file</span>=fopen(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"f.txt"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"r"</span>);   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//打开文件</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$con</span>=mysql_connect(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"localhost"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"root"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"root"</span>);  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//连接数据库</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$img</span>=imagecreate(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>);<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//图形画布</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

6.第二种特殊类型—空类型

NULL（NULL）：NULL是空类型，对大小写不敏感，NULL类型只有一个取值，表示一个变量没有值，当被赋值为NULL，或者尚未被赋值，或者被unset()，这三种情况下变量被认为为NULL。

例：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>  error_reporting(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//禁止显示PHP警告提示</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var</span>; var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var</span>); <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var1</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>; var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var1</span>); <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var2</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">NULL</span>; var_dump( <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var2</span>); <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var3</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"节日快乐！"</span>; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">unset</span>(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var3</span>); var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$var3</span>);<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

6

7.什么是常量

什么是常量？常量可以理解为值不变的量（如圆周率）；或者是常量值被定义后，在脚本的其他任何地方都不可以被改变。PHP中的常量分为自定义常量和系统常量（后续会详细介绍）。

自定义常量是根据我们开发的需要，而定义的常量，它通过使用PHP中的函数define()定义。（注：函数，我们可以理解为一个混凝土搅拌机，亦或是一个筛子，通过入口提供原料，然后出口产出结果，在函数中也允许入口不提供任何数据，出口也允许不返回任何值。）

define()函数的语法格式为：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">bool <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">define</span></span>(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">string</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$constant_name</span>, mixed <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$value</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">[</span>, <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$case_sensitive</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">]</span><span class="hljs-markup" style="box-sizing: border-box;">)</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

它有3个参数（也就是原料）：

第一个参数“constant_name”为必选参数，常量名称,即标志符，常量的命名规则与变量的一致，但是要注意哦，它可不带美元符号哦。第二个参数“value”为必选参数，它是常量的值。第三个参数“case_sensitive”为可选参数，指定是否大小写敏感，设定为true表示不敏感，一般不指定第三个参数的情况下，默认第三个参数的值为false。

（注： string表示参数类型为字符串类型，mixed表示参数类型可以接受为多种不同的类型，case_sensitive = true表示默认为布尔类型TRUE）

例：我们将圆周率定义为常量“PI”（通常常量都全用大写），取值为3.14：

<code class="hljs handlebars has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="xml" style="box-sizing: border-box;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PII"</span>;define(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3.14</span>);define(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3.14</span>);<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> PI;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> PII;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

7

8.系统常量

系统常量是PHP已经定义好的常量，我们可以直接拿来使用，常见的系统常量有：

（1）FILE :php程序文件名。它可以帮助我们获取当前文件在服务器的物理位置。

（2）LINE :PHP程序文件行数。它可以告诉我们，当前代码在第几行。

（3）PHP_VERSION:当前解析器的版本号。它可以告诉我们当前PHP解析器的版本号，我们可以提前知道我们的PHP代码是否可被该PHP解析器解析。

（4）PHP_OS：执行当前PHP版本的操作系统名称。它可以告诉我们服务器所用的操作系统名称，我们可以根据该操作系统优化我们的代码。

例：

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">__FILE__</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">__LINE__</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> PHP_VERSION;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> PHP_OS;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

8

9.常量如何取值

定义了常量，那么就要使用常量，那么如何获取常量值呢？

获取常量值的有两种方法取值。 
第一种是使用常量名直接获取值；例如计算圆周率的面积，如下：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>define(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3.14</span>);<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$r</span>=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$area</span> = PI*<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$r</span>*<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$r</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//计算圆的面积</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

第二种是使用constant()函数。它和直接使用常量名输出的效果是一样的，但函数可以动态的输出不同的常量，在使用上要灵活、方便，其语法格式如下：

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">mixed <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">constant</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">string</span> constant_name)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

第一个参数constant_name为要获取常量的名称，也可为存储常量名的变量。如果成功则返回常量的值，失败则提示错误信息常量没有被定义。（注：mixed表示函数返回值类型为多种不同的类型，string表示参数类型为字符串类型）

例：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><?</span>php <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">""</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//定义圆周率的两种取值</span><span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">define</span></span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI1"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3.14</span>);<span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">define</span></span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI2"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3.142</span>);<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//定义值的精度</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$height</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"中"</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//根据精度返回常量名，将常量变成了一个可变的常量</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$height</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"中"</span>){    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI1"</span>;}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$height</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"低"</span>){    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI2"</span>;}<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$r</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$area</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> constant(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span>)<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$r</span><span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$r</span>;echo <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$area</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span><span class="hljs-markup" style="box-sizing: border-box;"></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

9

10.如何判定常量是否被定义

如果常量被重复定义以后，PHP解析器会发出“Constant XXX already defined”的警告，提醒我们该常量已经被定义过。那么，在团队开发，或代码量很大的情况下，我们如何去判定一个常量是否被定义呢？

defined()函数可以帮助我们判断一个常量是否已经定义，其语法格式为：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">bool</span> defined(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">string</span> constants_name)</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

它只有参数constant_name，指的是要获取常量的名称，若存在则返回布尔类型true，否则返回布尔类型false; （注：bool表示函数返回值类型为布尔类型）

例：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><?php define(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI1"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3.14</span>);<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI1"</span>;<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$is1</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">defined</span>(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$p</span>);<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$is2</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">defined</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"PI2"</span>);var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$is1</span>);var_dump(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$is2</span>);?></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

12.运算符

PHP运算符一般分为算术运算符、赋值运算符、比较运算符、三元运算符、逻辑运算符、字符串连接运算符、错误控制运算符。

1.算术运算符：+，-，*，/，%

2.赋值运算符有两种：

(1)“=”：把右边表达式的值赋给左边的运算数。它将右边表达式值复制一份，交给左边的运算数。换而言之，首先给左边的运算数申请了一块内存，然后把复制的值放到这个内存中。

(2)“&”：引用赋值，意味着两个变量都指向同一个数据。它将使两个变量共享一块内存，如果这个内存存储的数据变了，那么两个变量的值都会发生变化。

例：

<code class="hljs handlebars has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="xml" style="box-sizing: border-box;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>     <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$a</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"我喜欢学习PHP！"</span>;    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$b</span> = <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$a</span>;    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$c</span> = &<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$a</span>;    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$a</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"我天天学习PHP！"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$b</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$c</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

11

3.比较运算符 
比较运算符主要是用于进行比较运算的，例如：等于、全等、不等、大于、小于。在PHP中常用的比较运算符如下表：

12

4.三元运算符 
(“?:”)三元运算符也是一个比较运算符，对于表达式(expr1)?(expr2):(expr3)，如果expr1的值为true，则此表达式的值为expr2，否则为expr3。

5.逻辑运算符 
逻辑运算符主用是进行逻辑运算的，例如：逻辑与、逻辑或、逻辑异或、逻辑非等，在PHP中常用的逻辑运算符如下表：

13

6.字符串连接运算符 
字符串连接运算符是为了将两个字符串进行连接，PHP中提供的字符串连接运算符有：

（1）连接运算符(“.”)：它返回将右参数附加到左参数后面所得的字符串。

（2）连接赋值运算符(“.=”)：它将右边参数附加到左边的参数后。

例：

<code class="hljs xml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="php" style="box-sizing: border-box;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>     <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$a</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"寂寞"</span>;    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$tip</span> = <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$a</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"欢迎你"</span>;    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$b</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"东边日出西边雨"</span>;        <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$b</span> .= <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">",道是无晴却有晴"</span>;    <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$c</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"东边日出西边雨"</span>;        <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$c</span> = <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$c</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">",道是无晴却有晴"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span>  <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$tip</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span>  <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$b</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span>  <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$c</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

12

7.错误控制运算符

PHP中提供了一个错误控制运算符“@”，对于一些可能会在运行过程中出错的表达式时，我们不希望出错的时候给客户显示错误信息，这样对用户不友好。于是，可以将@放置在一个PHP表达式之前，该表达式可能产生的任何错误信息都被忽略掉；

如果激活了track_error（这个玩意在php.ini中设置）特性，表达式所产生的任何错误信息都被存放在变量$php_errormsg中，此变量在每次出错时都会被覆盖，所以如果想用它的话必须尽早检查。

需要注意的是：错误控制前缀“@”不会屏蔽解析错误的信息，不能把它放在函数或类的定义之前，也不能用于条件结构例如if和foreach等。

例：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span>  ini_set(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'track_errors'</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);  <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$conn</span> = @mysql_connect(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"localhost"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"username"</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"password"</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"出错了，错误原因是："</span>.<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$php_errormsg</span>;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

13

13.结构

选择和循环结构和其他语言都差不多，不多介绍，只介绍不一样的：

循环结构之foreach循环语句

在PHP中foreach循环语句，常用于遍历数组，一般有两种使用方式:不取下标、取下标。

（1）只取值，不取下标

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">foreach</span> (数组 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> 值){<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//执行的任务</span>}<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

例：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$students</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">array</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2010'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'令狐冲'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2011'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'林平之'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2012'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'曲洋'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2013'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'任盈盈'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2014'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'向问天'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2015'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'任我行'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2016'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'冲虚'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2017'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'方正'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2018'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'岳不群'</span>,<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2019'</span>=><span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'宁中则'</span>,);<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//10个学生的学号和姓名，用数组存储</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//使用循环结构遍历数组,获取学号和姓名  </span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">foreach</span>(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$students</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$v</span>){     <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$v</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//输出（打印）姓名</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;}<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>

14

（2）同时取下标和值

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;"><?php</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">foreach</span> (数组 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> 下标 => 值){ <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//执行的任务</span>}<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

例：同上例：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">foreach</span>(<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$students</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">as</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$key</span> => <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$v</span>){     <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$key</span>.<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">":"</span>.<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">$v</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//输出（打印）学号：姓名</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">echo</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<br />"</span>;}<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">?></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><div class="save_code tracking-ad" data-mod="popu_249" style="box-sizing: border-box; position: absolute; height: 60px; right: 30px; top: 5px; color: rgb(255, 255, 255); cursor: pointer; z-index: 2;"><a target=_blank target="_blank" style="box-sizing: border-box; color: rgb(12, 137, 207);"><img src="http://static.blog.csdn.net/images/save_snippets.png" style="border: none; box-sizing: border-box;" alt="" /></a></div><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

15

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