Sanitize nested array structure

/**
 * Sanitizes input, converting types in $new to the type of that
 * same value in $old. Any keys not in $old will be stripped out.
 * @param array $new New settings.
 * @param array $old Old settings
 * @return array Sanitized version of $new, based on $old.
 */
private static function sanitizeSettings($new, $old = null) {
   $old = is_null($old) ? self::getSettings() : $old;
   $ret = $old;

   foreach ($old as $k => $v) {
      if (isset($new[$k])) {
         if (is_array($v)) {
            $ret[$k] = self::sanitizeSettings((array)$new[$k], $v);
         } elseif (settype($new[$k], gettype($v))) {
            $ret[$k] = $new[$k];
         }
      }
   }

   return $ret;
}