PHP: foreach - Manual

foreach

(PHP 4, PHP 5, PHP 7, PHP 8)

foreach 结构提供了一种遍历 array 和 Traversable object 的简便方式。当用于其他数据类型的变量，或未初始化的变量时，foreach 会触发错误。

foreach 可选择性的获取每个元素的 key：

foreach (iterable_expression as $value) {
    statement_list
}

foreach (iterable_expression as $key => $value) {
    statement_list
}

第一种格式遍历给定的 iterable_expression 迭代器。每次循环中，当前单元的值被赋给 $value。

第二种格式做同样的事，只除了当前单元的键名也会在每次循环中被赋给变量 $key。

注意 foreach 不会修改类似 current() 和 key() 函数所使用的数组内部指针。

还能够自定义遍历对象。

示例 #1 常见的 foreach 用法

<?php

/* 示例：仅有值 */
$array = [1, 2, 3, 17];

foreach ($array as $value) {
    echo "Current element of \$array: $value.\n";
}

/* 示例：key 和值 */
$array = [
    "one" => 1,
    "two" => 2,
    "three" => 3,
    "seventeen" => 17
];

foreach ($array as $key => $value) {
    echo "Key: $key => Value: $value\n";
}

/* 示例：多维 key-value 数组 */
$grid = [];
$grid[0][0] = "a";
$grid[0][1] = "b";
$grid[1][0] = "y";
$grid[1][1] = "z";

foreach ($grid as $y => $row) {
    foreach ($row as $x => $value) {
        echo "Value at position x=$x and y=$y: $value\n";
    }
}

/* 示例：动态数组 */
foreach (range(1, 5) as $value) {
    echo "$value\n";
}
?>

注意:
foreach 不支持使用 @ 运算符来抑制错误消息。

解包嵌套数组

(PHP 5 >= 5.5.0, PHP 7, PHP 8)

可以通过遍历数组中的数组，在 value 的位置使用数组解构（[]）或 list() 语言结构将嵌套数组解包到循环变量中。

注意: 请注意，通过 [] 进行数组解构仅在 PHP 7.1.0 及以上版本中可用。

在以下两个示例中，将设 $a 为嵌套数组的第一个元素，$b 将包含第二个元素：

<?php
$array = [
    [1, 2],
    [3, 4],
];

foreach ($array as [$a, $b]) {
    echo "A: $a; B: $b\n";
}

foreach ($array as list($a, $b)) {
    echo "A: $a; B: $b\n";
}
?>

以上示例会输出：

A: 1; B: 2
A: 3; B: 4

当提供的变量数量少于数组中的元素数量时，将会忽略多余的元素。类似地，可通过使用逗号跳过某些元素：

<?php
$array = [
    [1, 2, 5],
    [3, 4, 6],
];

foreach ($array as [$a, $b]) {
    // 注意此处没有 $c
    echo "$a $b\n";
}

foreach ($array as [, , $c]) {
    // 跳过 $a 和 $b
    echo "$c\n";
}
?>

以上示例会输出：

如果数组元素数量不足以填充 list()，将生成一条 notice 级别的错误消息。

<?php
$array = [
    [1, 2],
    [3, 4],
];

foreach ($array as [$a, $b, $c]) {
    echo "A: $a; B: $b; C: $c\n";
}
?>

以上示例会输出：

Notice: Undefined offset: 2 in example.php on line 7
A: 1; B: 2; C:

Notice: Undefined offset: 2 in example.php on line 7
A: 3; B: 4; C:

foreach 和引用

可以通过在 $value 前加上 &，就可以在循环中直接修改数组元素。此时，值将以引用的方式赋值。

<?php
$arr = [1, 2, 3, 4];
foreach ($arr as &$value) {
    $value = $value * 2;
}
// $arr is now [2, 4, 6, 8]
unset($value); // 断开与最后一个元素的引用
?>

警告

对数组最后一个元素的 $value 的引用在 foreach 循环结束后仍然存在。建议使用 unset() 将其销毁，否则将出现以下行为：

<?php
$arr = [1, 2, 3, 4];
foreach ($arr as &$value) {
    $value = $value * 2;
}
// $arr 现在是 [2, 4, 6, 8]

// 如果没有使用 unset(value)，$value 仍会引用最后一个元素：$arr[3]

foreach ($arr as $key => $value) {
    // $arr[3] 会随着 $arr 中的每个值而更新……
    echo "{$key} => {$value} ";
    print_r($arr);
}
// ……最终会复制倒数第二个值到最后一个值上
?>

以上示例会输出：

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 )

示例 #2 通过引用遍历常量数组的值

<?php
foreach ([1, 2, 3, 4] as &$value) {
    $value = $value * 2;
}
?>

参见

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用户贡献的备注 3 notes

down

Okafor Chiagozie ¶

3 years ago

An easier way to unpack nested array elements

$array = [
    [1, 2],
    [3, 4],
];

foreach ($array as [$a, $b]) {
    echo "A: $a; B: $b\n";
}

down

renatoaraujoleal at gmail dot com ¶

6 months ago

<?php
$array = [
    [1, 2, 3],
    [3, 4, 6],
];

foreach ($array as [$a, $b]) {
    // Observe que não existe $c aqui.
    echo "$a $b\n";
}

foreach ($array as [, , $c]) {
    // Pulando $a e $b
    echo "$c\n";
}
?>

I would like to correct this example above!
The answer of this algorithm is:

1 2
3 4
3
6

down

sunfundev at gmail dot com ¶

30 days ago

Definitely relevant for PHP 7+

1. You can't change array during iteration

Foreach WILL NOT LOOP through new values added to the array 
<?php
while inside the loop.
$a = [1, 2, 3];
foreach ($a as $k => $v) {
    echo $v;

    if ($v === 2) {
        $v[] = 4;
    }
}
?>
Output: 123
But the original array was modified: [1, 2, 3, 4]

Foreach WILL LOOP through values deleted from the array while inside the loop.
<?php
$a = [1, 2, 3];
foreach ($a as $k => $v) {
    echo $v;

    if ($v === 2) {
        unset($a[2]);
    }
}
?>
Output: 123
But the original array was modified: [1, 2]

2. But If you iterate by reference using foreach ($arr as &$v) then $arr is turned into a reference and you can change it during iteration

Foreach WILL LOOP through new values added to the array while inside the loop.
<?php
$a = [1, 2, 3];
foreach ($v as &$v) {
    echo $v;

    if ($v === 2) {
        $v[] = 4;
    }
}
?>
Output: 1234

Foreach WILL NOT LOOP through values deleted from the array while inside the loop.
<?php
$a = [1, 2, 3];
foreach ($a as $k => &$v) {
    echo $v;

    if ($v === 2) {
        unset($a[2]);
    }
}
?>
Output: 12