Description:
------------
---
From manual page: https://php.net/function.array-key-exists
---

array_key_exists() returns false when checking for a key that is boolean false or true, yet the value can be accessed by $thearray[false] & array_keys($thearray) includes the boolean true/false

null is not int|string, but works without issue, so I think that part is a documentation issue.

Test script:
---------------
$data = [false=>'happy', null=>'sad', true=>'also happy'];

var_dump(array_key_exists(true, $data)); // false, PHP Warning
var_dump(array_key_exists(false, $data)); // false, PHP Warning
var_dump(array_key_exists(null, $data)); // true, No Warning

var_dump($data[false]); // happy

var_dump(
    in_array(false,array_keys($data))
); // true 

var_dump(
    in_array(true,array_keys($data))
); // true 
var_dump(
    in_array(null,array_keys($data))
); // true 


Expected result:
----------------
var_dump output: 

true
true
true
"happy"
true
true
true

(and no errors)

Actual result:
--------------
var_dump output: 

false
false
true
"happy"
true
true
true

PHP Warning:  array_key_exists(): The first argument should be either a string or an integer
PHP Warning: (same warning again)