array_key_exists is not working

array_key_exists is not working for large multidimensional array. For ex

$arr = array(
    '1' => 10,
    '2' => array(
        '21' => 21,
        '22' => 22,
        '23' => array(
            'test' => 100,
            '231' => 231
        ),
    ),
    '3' => 30,
    '4' => 40
);

array_key_exists('test',$arr) returns 'false' but it works with some simple arrays.

array_key_exists does NOT work recursive (as Matti Virkkunen already pointed out). Have a look at the PHP manual, there is the following piece of code you can use to perform a recursive search:

<?php
function array_key_exists_r($needle, $haystack)
{
    $result = array_key_exists($needle, $haystack);
    if ($result) return $result;
    foreach ($haystack as $v) {
        if (is_array($v)) {
            $result = array_key_exists_r($needle, $v);
        }
        if ($result) return $result;
    }
    return $result;
}