Void Parameter In array_keys()¶
<?php
$array = [3 => null, 4 => 4];
var_dump(array_keys($array, null));
// 3
var_dump(array_keys($array /*, void */));
// 3, 4
There is a ‘void’ parameter in PHP. It is the second argument of array_keys().
That second parameter is often omitted (and unknown).
If present, it is typed ‘mixed’ to allow any value to be searched (here, null).
If absent, array_keys() returns ALL keys. When absent, it is not null, nor any other type. The last one possible is ‘void’.
Type is then : void|mixed.
See Also¶
null as a parameter [Try me]
PHP Features¶
Last updated: 14 July 2026