public function read($session_id)
    {
        //string casting is a fix for PHP 7
        //when strict type are enable
        return (string) $this->pdo->queryWithParam(
            'SELECT session_data FROM session WHERE session_id = :session_id',
            [[':session_id', $session_id, \PDO::PARAM_STR]]
        )->fetchColumn();
    }