code: static function Db() : Connection {
          if (!isset(self::$_connection)) {
              [$host, $port, $user, $password, $db_name, $charset] = self::$_config;
              self::$_connection = new Connection($host, $port, $user, $password, $db_name, $charset);
          }
          return self::$_connection;
      }
docs: 获取当前实例的数据库连接
  
  @return Connection