PHP Manual: array_walk

Description

function array_walk_recursive(array|object &$array, callable $callback, mixed $arg = null): true

Applies the user-defined callback function to each element of the array. This function will recurse into deeper arrays.

Passing an object as the array is allowed, and will be treated as if it had been cast to an array first. Regardless of whether the function started with an array or an object, it will not recurse into inner objects.

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

Return Values

Always returns true.

Changelog

Version	Description
8.2.0	The return type is `true` now; previously, it was bool.

Examples

Example #1 array_walk_recursive() example

<?php
$sweet = array('a' => 'apple', 'b' => 'banana');
$fruits = array('sweet' => $sweet, 'sour' => 'lemon');

function test_print($item, $key)
{
    echo "$key holds $item\n";
}

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

The above example will output:

a holds apple
b holds banana
sour holds lemon

You may notice that the key 'sweet' is never displayed. Any key that holds an array will not be passed to the function.

array_walk_recursive() can also be used to update values in place by passing the callback parameter by reference.

Example #2 Updating values in place

<?php
$sweet = ['a' => 'apple', 'b' => 'banana'];
$fruits = ['sweet' => $sweet, 'sour' => 'lemon'];

function test_update(&$item)
{
    $item = strtoupper($item);
}

function test_print($item, $key)
{
    echo "$key holds $item\n";
}

array_walk_recursive($fruits, 'test_update');
array_walk_recursive($fruits, 'test_print');
?>