PHP5+ function: array_diff_key not supported in PHP4

The logic of the patch submitted here #224359: Fatal error: Call to undefined function: array_diff_key() needs to be applied to notifications_ui.module if you are using PHP4.

Basically, add the following function to this module:

if (!function_exists('array_diff_key')) {
  function array_diff_key() {
    $arrs = func_get_args();
    $result = array_shift($arrs);
    foreach ($arrs as $array) {
      foreach ($result as $key => $v) {
        if (array_key_exists($key, $array)) {
          unset($result[$key]);
        }
      }
    }
    return $result;
   }
}