PHP array_walk_recursive Function

last modified March 13, 2025

The PHP array_walk_recursive function applies a user function recursively to every element of an array. It's useful for nested arrays.

Basic Definition

array_walk_recursive processes arrays recursively, applying a callback to each non-array element. It preserves array keys during traversal.

Syntax: array_walk_recursive(array &$array, callable $callback, mixed $userdata = null): bool. The callback receives value and key parameters.

Basic array_walk_recursive Example

This example demonstrates printing all values from a nested array structure.

basic_array_walk_recursive.php

<?php

declare(strict_types=1);

$data = [
    'name' => 'John',
    'contacts' => [
        'email' => 'john@example.com',
        'phone' => '123456789'
    ],
    'age' => 30
];

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

The callback function receives each key-value pair. Nested arrays are processed recursively, but their parent keys aren't passed to the callback.

Modifying Array Values

This example shows how to modify array values in place using references.

modify_values.php

<?php

declare(strict_types=1);

$numbers = [
    'a' => 1,
    'b' => [2, 3],
    'c' => 4
];

array_walk_recursive($numbers, function(&$value, $key) {
    $value *= 2;
});

print_r($numbers);

By passing the value by reference (&$value), we can modify the original array elements. All numeric values are doubled in this example.

Using User Data Parameter

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

user_data.php

<?php

declare(strict_types=1);

$products = [
    'item1' => ['price' => 100, 'quantity' => 2],
    'item2' => ['price' => 200, 'quantity' => 1]
];

$discount = 0.1; // 10% discount

array_walk_recursive($products, function(&$value, $key, $discount) {
    if ($key === 'price') {
        $value *= (1 - $discount);
    }
}, $discount);

print_r($products);

The discount value is passed as user data and applied only to price fields. This demonstrates selective modification based on both key and user data.

Counting Array Elements

This example counts all non-array elements in a multidimensional array.

counting_elements.php

<?php

declare(strict_types=1);

$data = [
    'a' => 1,
    'b' => [2, 3, [4, 5]],
    'c' => 6
];

$count = 0;
array_walk_recursive($data, function($value) use (&$count) {
    $count++;
});

echo "Total elements: $count";

Using a closure with variable binding, we increment a counter for each non-array element. The result shows the total count of leaf nodes.

Building a Flat Array

This example collects all values from a nested array into a flat array.

flatten_array.php

<?php

declare(strict_types=1);

$nested = [
    'a' => 1,
    'b' => [
        'c' => 2,
        'd' => [3, 4]
    ],
    'e' => 5
];

$flat = [];
array_walk_recursive($nested, function($value) use (&$flat) {
    $flat[] = $value;
});

print_r($flat);

By appending each value to an external array, we create a flattened version. This technique is useful when you need to process all values uniformly.

Best Practices

Reference Parameters: Use &$value to modify array elements.
Key Awareness: Check $key when you need conditional logic.
User Data: Pass additional data via the third parameter.
Closures: Use 'use' keyword to access external variables.
Performance: Avoid complex operations in large arrays.

Source

PHP array_walk_recursive Documentation

This tutorial covered the PHP array_walk_recursive function with practical examples showing its usage for processing nested array structures.

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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