PHP array_walk Function

last modified March 13, 2025

The PHP array_walk function applies a user-defined callback to each element of an array. It's useful for processing array elements without creating a new array.

Basic Definition

The array_walk function iterates through each array element and applies a callback function to it. The original array is modified by reference.

Syntax: array_walk(array &$array, callable $callback, mixed $userdata = null): bool. The callback receives both value and key by default. Returns true on success.

Basic array_walk Example

This demonstrates modifying each element in an array by squaring the numbers.

basic_array_walk.php

<?php

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

function square(&$value, $key) {
    $value = $value * $value;
}

array_walk($numbers, 'square');

print_r($numbers);

The callback modifies each element by reference. The original array is changed to contain squared values. Note the & before $value in the callback.

Using array_walk with Keys

This example shows how to access both keys and values in the callback.

array_walk_keys.php

<?php

$fruits = [
    'a' => 'apple',
    'b' => 'banana',
    'c' => 'cherry'
];

function printPair($value, $key) {
    echo "$key: $value\n";
}

array_walk($fruits, 'printPair');

The callback receives both key and value parameters. This allows processing associative arrays where keys are meaningful. Output shows each key-value pair.

Using User Data with array_walk

The third parameter allows passing additional data to the callback function.

array_walk_userdata.php

<?php

$prices = [10, 20, 30];
$discount = 0.2; // 20% discount

function applyDiscount(&$price, $key, $discount) {
    $price = $price * (1 - $discount);
}

array_walk($prices, 'applyDiscount', $discount);

print_r($prices);

The discount rate is passed as user data. The callback applies this discount to each price. This demonstrates how to share data across all callback calls.

Modifying Array Elements by Reference

This example shows how to modify array elements directly in the callback.

array_walk_reference.php

<?php

$names = ['alice', 'bob', 'charlie'];

function capitalize(&$name) {
    $name = ucfirst($name);
}

array_walk($names, 'capitalize');

print_r($names);

The callback capitalizes each name by modifying the value through reference. The original array is updated with capitalized names after array_walk executes.

Using Anonymous Functions with array_walk

Modern PHP allows using anonymous functions for more concise code.

array_walk_anonymous.php

<?php

$temperatures = [22.5, 19.3, 25.1, 18.7];

array_walk($temperatures, function(&$temp) {
    $temp = round($temp);
});

print_r($temperatures);

An anonymous function rounds each temperature. This approach keeps the logic close to where it's used, improving code readability for simple operations.

Best Practices

Reference Parameters: Use & to modify array elements directly.
Type Safety: Add type hints for robust callback functions.
Performance: Avoid complex operations in large array processing.
Readability: Use named functions for complex transformations.
Return Values: Remember array_walk returns bool, not the array.

Source

PHP array_walk Documentation

This tutorial covered the PHP array_walk function with practical examples showing its usage for array processing scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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