PHP坑之：foreach &

代码如下：

$a = array('a','b','c','d');foreach ($a as &$item){};print_r($a);foreach ($a as $item){};print_r($a);

输出结果：

Array(    [0] => a    [1] => b    [2] => c    [3] => d)Array(    [0] => a    [1] => b    [2] => c    [3] => c)

问题：两次foreach什么也没做，其中一个使用了&，结果导致数组内容被改变，是为什么呢？

打印详情输出一下：

$a = array('a','b','c','d');foreach ($a as &$item){};print_r($a);foreach ($a as $key => $item) {echo "{$key} => {$item} ";print_r($a);}

输出：

Array(    [0] => a    [1] => b    [2] => c    [3] => d)0 => a Array(    [0] => a    [1] => b    [2] => c    [3] => a)1 => b Array(    [0] => a    [1] => b    [2] => c    [3] => b)2 => c Array(    [0] => a    [1] => b    [2] => c    [3] => c)3 => c Array(    [0] => a    [1] => b    [2] => c    [3] => c)

原因及修正：

$a = array('a','b','c','d');foreach ($a as &$item){};//此时$item指向$a[3],即'd'print_r($a);//php官网给出的正确做法是//unset($item);//foreach 下标从0到3,每次循环时取下标赋值给$key,取value赋值给$item,而$item经过上面的循环指向$a[3]foreach ($a as $key => $item) {//每次都修改$item的值(也就是$a[3]的值)为当前下标所指的值：a,b,c;直到最后第n个和第n-1个值相等echo "{$key} => {$item} ";print_r($a);}