$str_2 = "str_1 : ${str_1}2 "; //引用的变量名后，多了个字符2 即$str_12

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title> str_var.php </title>
<meta charset="UTF-8">
<meta name="Generator" content="EditPlus">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
</head>

<body>
<?php
$str_1 = "我是变量的值!";
$str_2 = "str_1 : $str_1 "; //双引号字符串中包含变量$str_1
echo $str_2 . '<hr>';

$str_1 = '我是变量的值';
$str_2 = 'str_1 : $str_1 '; //单引号中包含字符串的值
echo $str_2 . '<hr>';

$str_1 = "我是变量的值!";
$str_2 = "str_1 : $str_12 "; //引用的变量名后，多了个字符2 即$str_12
echo $str_2 . '<hr>';

$str_1 = "我是变量的值!";
$str_2 = "str_1 : ${str_1}2 "; //引用的变量名后，多了个字符2 即$str_12
echo $str_2 . '<hr>';

?>
</body>
</html>

str_1 : 我是变量的值!

str_1 : $str_1

( ! ) Notice: Undefined variable: str_12 in D:\LearnWebDevelop\php\MyPhpExse\str_var.php on line23Call Stack#TimeMemoryFunctionLocation10.0006367656{main}( )..\str_var.php:0

str_1 :

str_1 : 我是变量的值!2

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$str_2 = "str_1 : ${str_1}2 <br>"; //引用的变量名后，多了个字符2 即$str_12