/**
 * http://php.net/array_diff_key recursive implementation.
 * 
 * @todo Support variadic input.
 * @param array $arr1 The array with master keys to check.
 * @param array $arr2 An array to compare keys against.
 * @return array
 */
function array_diff_key_recursive (array $arr1, array $arr2) {
    $diff = array_diff_key($arr1, $arr2);
    $intersect = array_intersect_key($arr1, $arr2);
    
    foreach ($intersect as $k => $v) {
        if (is_array($arr1[$k]) && is_array($arr2[$k])) {
            $d = array_diff_key_recursive($arr1[$k], $arr2[$k]);
            
            if ($d) {
                $diff[$k] = $d;
            }
        }
    }
    
    return $diff;
}