PHP Manual: array_key

array_key_exists() returns true if the given key is set in the array. key can be any value possible for an array index.

Returns true on success or false on failure.

Note:
array_key_exists() will search for the keys in the first dimension only. Nested keys in multidimensional arrays will not be found.

Version	Description
8.5.0	Using null in the `key` parameter is deprecated, use an empty string instead.
8.0.0	The `key` parameter now accepts bool, float, int, null, resource, and string as arguments.
8.0.0	Passing an object to the `array` parameter is no longer supported.
7.4.0	Passing an object to the `array` parameter has been deprecated. Use property_exists() instead.

Example #1 array_key_exists() example

<?php
$searchArray = ['first' => 1, 'second' => 4];
var_dump(array_key_exists('first', $searchArray));
?>

The above example will output:

bool(true)

Example #2 array_key_exists() vs isset()

isset() does not return true for array keys that correspond to a null value, while array_key_exists() does.

<?php
$searchArray = ['first' => null, 'second' => 4];

var_dump(isset($searchArray['first']));
var_dump(array_key_exists('first', $searchArray));
?>

The above example will output:

bool(false)
bool(true)

array_key_exists