PHP array_keys() Function: Usage, Parameters, Return Values, and Advanced Examples

array_keys() is a commonly used PHP function that returns all the keys of an array.

Function signature: array_keys($array, $search_value = null, $strict = false)

Parameters:

$array (required): the input array.

$search_value (optional): value to search for.

$strict (optional): if true, performs strict type comparison; default false.

Return value: If $search_value is omitted, returns an array of all keys. If provided, returns keys whose values match $search_value .

Basic usage example:

<?php
$array = array('a' => 'apple', 'b' => 'banana', 'c' => 'cherry', 'd' => 'durian');
$keys = array_keys($array);
print_r($keys);
$banana_keys = array_keys($array, 'banana');
print_r($banana_keys);
?>

Output:

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

Advanced usage – first matching key:

<?php
$array = array('a' => 'apple', 'b' => 'banana', 'c' => 'cherry', 'd' => 'durian', 'e' => 'banana');
$banana_keys = array_keys($array, 'banana');
if (count($banana_keys) > 0) {
    $first_banana_key = current(array_slice($banana_keys, 0, 1));
    echo 'First matching key: ' . $first_banana_key;
}
?>

Output:

First matching key: b

Strict comparison example:

<?php
$array = array('1' => 'apple', '2' => 'banana', '3' => 'cherry');
// Without strict comparison
$keys = array_keys($array, '2');
echo 'Without strict: ';
print_r($keys);
// With strict comparison
$keys = array_keys($array, '2', true);
echo 'With strict: ';
print_r($keys);
?>

Output:

Without strict: Array ( [0] => 2 )
With strict: Array ( )

Summary: array_keys() efficiently retrieves keys from an array, supports optional value searching and strict type comparison, but should be used cautiously with large arrays to avoid memory or performance issues.