#2 PHP 中 foreach 循环赋值给引用的一个特性（坑）
PHP 2016-10-17复现方法
创建一个存文本文件，命名为：test.php，内容如下：
<?php
$menu = [
1 => ['id' => 1, 'name' => 'a', 'child' => []],
2 => ['id' => 2, 'name' => 'b', 'child' => []],
];
foreach ($menu as &$submenu) {
$submenu['class'] = empty($submenu['child']) ? '' : 'hasChild';
}
print_r($menu);
print str_repeat('-', 80) . "\n";
foreach ($menu as $submenu) {
print_r($submenu);
}
?>
命令行中运行：
php -f test.php
得到以下输出：
Array
(
[1] => Array
(
[id] => 1
[name] => a
[child] => Array
(
)
[class] =>
)
[2] => Array
(
[id] => 2
[name] => b
[child] => Array
(
)
[class] =>
)
)
--------------------------------------------------------------------------------
Array
(
[id] => 1
[name] => a
[child] => Array
(
)
[class] =>
)
Array
(
[id] => 1
[name] => a
[child] => Array
(
)
[class] =>
)
可以发现，第二个 foreach 中的输出是错误的。
来自官网的说明
<?php
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
$value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
// without an unset($value), $value is still a reference to the last item: $arr[3]
foreach ($arr as $key => $value) {
// $arr[3] will be updated with each value from $arr...
echo "{$key} => {$value} ";
print_r($arr);
}
// ...until ultimately the second-to-last value is copied onto the last value
// output:
// 0 => 2 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 2 )
// 1 => 4 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 4 )
// 2 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
// 3 => 6 Array ( [0] => 2, [1] => 4, [2] => 6, [3] => 6 )
?>
参考
- PHP 官方文档，PHP: foreach - Manual