private function cleanExpired()
    {
        // 10% chance to clean the database of expired sessions
        if (mt_rand(1, 10) == 1) {
            $expiredTime = $this->now - $this->secondsUntilExpiration;
            $stmt = $this->db->prepare("DELETE FROM {$this->tableName} WHERE time_updated < {$expiredTime}");
            $stmt->execute();
        }
    }