public function getDriver()
    {
        if (null != $this->driver) {
            return $this->driver;
        }

        switch($this->config['driver']) {
            case "file":
                $this->driver   = $this->createFileDriver();
                break;
            case "memcached":
                $this->driver   = $this->createMemcacheDriver();
                break;
            case "redis":
                $this->driver   = $this->createRedisDriver();
                break;
            default:
                throw new SessionException("Driver [{$this->config['driver']}] not supported.");
        }

        return $this->driver;
    }