public static function connect()
 {
     if (!isset(self::$con) || null === self::$con) {
         self::$con = new MySQLi(SERVER, USER, PASS, DB);
         self::$con->set_charset('utf8');
         if (mysqli_connect_error()) {
             printf("connect failed %d", mysqli_connect_error);
             return false;
         }
     }
     return self::$con;
 }