PHP array_keys Function

last modified March 13, 2025

The PHP array_keys function returns all the keys or a subset of keys from an array. It’s useful for extracting and working with array keys.

Basic Definition

The array_keys function returns all the keys of an array. It can also return keys for a specific value when the search_value parameter is used.

Syntax: array_keys(array $array, mixed $search_value = null, bool $strict = false): array. The function returns a new array containing the keys.

Basic array_keys Example

This shows how to extract all keys from a simple associative array.

basic_array_keys.php

<?php

$user = [ ’name’ => ‘John’, ‘age’ => 30, ’email’ => ‘john@example.com’ ];

$keys = array_keys($user);

print_r($keys);

This code outputs all keys from the $user array. The result will be: Array ( [0] => name [1] => age [2] => email ).

Searching for Specific Values

Find all keys that have a specific value in an array.

search_values.php

<?php

$colors = [ ‘red’ => ‘#FF0000’, ‘green’ => ‘#00FF00’, ‘blue’ => ‘#0000FF’, ‘dark_red’ => ‘#FF0000’ ];

$redKeys = array_keys($colors, ‘#FF0000’);

print_r($redKeys);

This finds all keys with the value ‘#FF0000’. The output will be: Array ( [0] => red [1] => dark_red ).

Using Strict Comparison

Demonstrate the difference between loose and strict comparison when searching.

strict_comparison.php

<?php

$data = [ ‘a’ => ‘1’, ‘b’ => 1, ‘c’ => ‘1.0’, ’d’ => 1.0 ];

$looseMatch = array_keys($data, 1); $strictMatch = array_keys($data, 1, true);

print_r($looseMatch); print_r($strictMatch);

With loose comparison, all elements match 1. With strict comparison, only the integer 1 matches. This shows the importance of the strict parameter.

Working with Numeric Arrays

Even numeric arrays have keys, which array_keys can extract.

numeric_array.php

<?php

$fruits = [‘apple’, ‘banana’, ‘cherry’]; $keys = array_keys($fruits);

print_r($keys);

This demonstrates that numeric arrays have sequential numeric keys. The output will be: Array ( [0] => 0 [1] => 1 [2] => 2 ).

Multidimensional Arrays

Extract keys from a multidimensional array structure.

multidimensional_array.php

<?php

$users = [ ‘user1’ => [’name’ => ‘Alice’, ‘age’ => 25], ‘user2’ => [’name’ => ‘Bob’, ‘age’ => 30], ‘user3’ => [’name’ => ‘Charlie’, ‘age’ => 35] ];

$userKeys = array_keys($users);

print_r($userKeys);

This extracts the top-level keys from a multidimensional array. The output will be: Array ( [0] => user1 [1] => user2 [2] => user3 ).

Best Practices

• Memory Usage: Be mindful when working with large arrays.

• Strict Mode: Use strict comparison for precise matching.

• Key Types: Remember keys can be integers or strings.

• Performance: Consider alternatives for simple iterations.

Source

PHP array_keys Documentation

This tutorial covered the PHP array_keys function with practical examples showing its usage for various array key extraction 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.

List all PHP Array Functions.