<?php
# array_keys() also return the key if it's boolean but the boolean will return as 1 or 0. It will return empty if get NULL value as key. Consider the following array:
$a = array(
"first_index" => "This is the first element",
true => 3,
false => 2,
4.5 => 'Something',
"08" => 5,
"8" => 6,
NULL => 'Null key'
);
print_r(array_keys($a));
Array
(
[0] => first_index
[1] => 1
[2] => 0
[3] => 4
[4] => 08
[5] => 8
[6] =>
)
?>