How to Use array_keys in PHP: Syntax and Examples

In PHP, array_keys returns all the keys from an array. You can optionally pass a value to get only keys matching that value. It helps you find keys quickly without looping manually.

Syntax

The array_keys function has this syntax:

• array_keys(array $array, mixed $search_value = null, bool $strict = false): array

Parameters:

• $array: The input array to get keys from.
• $search_value (optional): If given, only keys with this value are returned.
• $strict (optional): If true, uses strict type comparison when searching.

The function returns an array of keys.

php

array_keys(array $array, mixed $search_value = null, bool $strict = false): array

Example

This example shows how to get all keys from an array and how to get keys for a specific value.

php

<?php
$fruits = [
    'a' => 'apple',
    'b' => 'banana',
    'c' => 'cherry',
    'd' => 'banana'
];

// Get all keys
$allKeys = array_keys($fruits);
print_r($allKeys);

// Get keys where value is 'banana'
$bananaKeys = array_keys($fruits, 'banana');
print_r($bananaKeys);
?>

Output

Array
(
    [0] => a
    [1] => b
    [2] => c
    [3] => d
)
Array
(
    [0] => b
    [1] => d
)

Common Pitfalls

One common mistake is forgetting that array_keys returns an array of keys, not a single key. Also, when searching for a value, if you don't set $strict to true, PHP uses loose comparison which can cause unexpected matches.

Example of loose vs strict search:

php

<?php
$array = [
    0 => '0',
    1 => 0,
    2 => false
];

// Loose search (default)
$keysLoose = array_keys($array, 0);
print_r($keysLoose);

// Strict search
$keysStrict = array_keys($array, 0, true);
print_r($keysStrict);
?>

Output

Array
(
    [0] => 0
    [1] => 1
    [2] => 2
)
Array
(
    [0] => 1
)

Quick Reference

Parameter	Description
$array	The input array to extract keys from.
$search_value (optional)	Value to filter keys by matching values.
$strict (optional)	Use strict comparison (===) when searching values.

Key Takeaways

Use array_keys to get all keys or keys matching a specific value from an array.

Passing a value to array_keys filters keys to those with that value.

Set the strict parameter to true to avoid loose comparison surprises.

array_keys always returns an array of keys, even if only one key matches.

It's a quick way to find keys without writing loops.