protected static function _sortObjects(&$a, &$b)
  {
    $key = self::$sortKey;

    for($i=0, $count=count($key); $i<$count; $i++){
      if ( isset(self::$sortDirection[$i]) ){
        $direction = self::$sortDirection[$i];
      }

      if ( isset(self::$sortCase[$i]) ){
        $caseSensitive = self::$sortCase[$i];
      }

      if ( isset(self::$sortLocale[$i]) ){
        $locale = self::$sortLocale[$i];
      }

      $va = $a->$key[$i];
      $vb = $b->$key[$i];

      if ( (is_bool($va) or is_numeric($va)) and (is_bool($vb) or is_numeric($vb)) ){
        $cmp = $va - $vb;
      }
      elseif ( $caseSensitive ){ 
        $cmp = strcmp($va, $vb, $locale);
      }
      else {
        $cmp = strcasecmp($va, $vb, $locale);
      }

      if ( $cmp > 0 ){

        return $direction;
      }

      if ( $cmp < 0 ){
        return -$direction;
      }
    }

    return 0;
  }