array_walk

Apply a user supplied function to every member of an array

&reftitle.description; `true` `array_walk` ``array``object```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.

&reftitle.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.

  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.

`arg`

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

&reftitle.returnvalues;

&return.true.always;

&reftitle.errors;

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`.

&reftitle.changelog;

&Version; | &Description;
&return.type.true;
8.0.0 | If `callback` expects the second or third parameter to be passed by reference, this function will now emit an `E_WARNING`.

&reftitle.examples;

### `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');
    ?>

&example.outputs;

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

### `array_walk` example using anonymous function

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

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

    ?>

&example.outputs;

    0 => a
    1 => b
    2 => c

&reftitle.seealso;

- `array_walk_recursive`
- `iterator_apply`
- `list`
- `each`
- `call_user_func_array`
- `array_map`
- &foreach;