public function close()
    {
        if ($this->gcCalled) {
            $this->gcCalled = false;

            // delete the session records that have expired
            $sql = "DELETE FROM $this->table WHERE $this->lifetimeCol + $this->timeCol < :time";

            $stmt = $this->db->query($sql)
                        ->bind(':time', time())
                        ->execute();
        }

        return true;
    }