public static function db($set = false)
 {
     if ($set instanceof PDO) {
         self::$db = $set;
         self::$dbtype = self::DBTYPE_PDO;
         return true;
     }
     if ($set instanceof MySQLi) {
         self::$db = $set;
         self::$dbtype = self::DBTYPE_MYSQLI;
         return true;
     }
     return false;
 }