(PHP 4, PHP 5, PHP 7)

array_walkApply a user supplied function to every member of an array


array_walk ( array|object &$array , callable $callback [, mixed $userdata = null ] ) : bool

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.



The input array.


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


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.


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.


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

Return Values

Returns true.


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). Previously, if the callback function required more than 2 parameters, an error of level E_WARNING would be generated each time array_walk() calls callback.


Example #1 array_walk() example

$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<br />\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

See Also

© 1997–2020 The PHP Documentation Group
Licensed under the Creative Commons Attribution License v3.0 or later.