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.

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