PHP Manual: Apply a user supplied function to every member of an array

Description

true array_walk(arrayobject &$array, callable $callback, mixed $arg = null)

Applies the user-defined callback function to each element of the array array.

array_walk is not affected by the internal array pointer of array. array_walk will walk through the entire array regardless of pointer position.

Parameters

array

The input array.

callback

Typically, callback takes on two parameters. The array parameter's value being the first, and the key/index second.

Note:
If callback needs to be working with the actual values of the array, specify the first parameter of callback as a reference. Then, any changes made to those elements will be made in the original array itself.

Note:
Many internal functions (for example strtolower) will throw a warning if more than the expected number of argument are passed in and are not usable directly as a callback.

Only the values of the array may potentially be changed; its structure cannot be altered, i.e., the programmer cannot add, unset or reorder elements. If the callback does not respect this requirement, the behavior of this function is undefined, and unpredictable.

arg

If the optional arg parameter is supplied, it will be passed as the third parameter to the callback.

Return Values

Always returns true.

Errors/Exceptions

As of PHP 7.1.0, an ArgumentCountError will be thrown if the callback function requires more than 2 parameters (the value and key of the array member), or more than 3 parameters if the arg is also passed. Previously, in this case an error of level E_WARNING would be generated each time array_walk calls callback.

Changelog

Version	Description
8.2.0	The return type is `true` now; previously, it was bool.
8.0.0	If `callback` expects the second or third parameter to be passed by reference, this function will now emit an `E_WARNING`.

Examples

Example #1 array_walk example

<?php
$fruits = array("d" => "lemon", "a" => "orange", "b" => "banana", "c" => "apple");

function test_alter(&$item1, $key, $prefix)
{
    $item1 = "$prefix: $item1";
}

function test_print($item2, $key)
{
    echo "$key. $item2\n";
}

echo "Before ...:\n";
array_walk($fruits, 'test_print');

array_walk($fruits, 'test_alter', 'fruit');
echo "... and after:\n";

array_walk($fruits, 'test_print');
?>

The above example will output:

Before ...:
d. lemon
a. orange
b. banana
c. apple
... and after:
d. fruit: lemon
a. fruit: orange
b. fruit: banana
c. fruit: apple

Example #2 array_walk example using anonymous function

<?php
$elements = ['a', 'b', 'c'];

array_walk($elements, function ($value, $key) {
  echo "{$key} => {$value}\n";
});

?>