getLogoutUrl不与facebook graphp api使用php codeigniter工作

我的facebook php sdk getLogoutUrl不工作，当我点击我的注销url。它带我回到我给定的重定向url，但它不会破坏我的facebook会话，我仍然可以看到我的var_dump($fb_data)数组在我的页面和注销url。

这是我的代码，我使用codeigniter我的lib_login库函数facebook代码

 public function facebook()
    {
        $facebook_default_scope = explode(',', $this->ci->config->item("facebook_default_scope"));
        $facebook_app_id = $this->ci->config->item("facebook_app_id");
        $facebook_api_secret = $this->ci->config->item("facebook_api_secret");
        // init app with app id and secret
        FacebookSession::setDefaultApplication($facebook_app_id, $facebook_api_secret);
        // login helper with redirect_uri
        $helper = new FacebookRedirectLoginHelper(site_url('login/facebook'));
        // see if a existing session exists
        if (isset($_SESSION) && isset($_SESSION['fb_token'])) {
            // create new session from saved access_token
            $session = new FacebookSession($_SESSION['fb_token']);
            // validate the access_token to make sure it's still valid
            try {
                if (!$session->validate()) {
                    $session = null;
                }
            } catch (Exception $e) {
                // catch any exceptions
                $session = null;
            }
        }
        if (!isset($session) || $session === null) {
            // no session exists
            try {
                $session = $helper->getSessionFromRedirect();
            } catch(FacebookRequestException $ex) {
                // When Facebook returns an error
                // handle this better in production code
                print_r($ex);
            } catch(Exception $ex) {
                // When validation fails or other local issues
                // handle this better in production code
                print_r($ex);
            }
        }
        // see if we have a session
        if (isset($session)) {
            // save the session
            $_SESSION['fb_token'] = $session->getToken();
            // create a session using saved token or the new one we generated at login
            $session = new FacebookSession($session->getToken());
            // graph api request for user data
            $request = new FacebookRequest($session, 'GET', '/me?fields=id,name,accounts{access_token,category,name,id,perms},permissions');
            $response = $request->execute();
            // get response
            $graphObject = $response->getGraphObject()->asArray();
            $logoutUrl = site_url('login');
            $fb_data = array(
                'me' => $graphObject,
                'loginUrl' => $helper->getLoginUrl($facebook_default_scope),
                'logoutUrl' => $helper->getLogoutUrl($session,$logoutUrl),
           );
            $this->ci->session->set_userdata('fb_data', $fb_data);
        } else {
            $fb_data = array(
                'me' => null,
                'loginUrl' => $helper->getLoginUrl($facebook_default_scope),
                'logoutUrl' => $helper->getLogoutUrl($session,$logoutUrl),
           );
            $this->ci->session->set_userdata('fb_data', $fb_data);
        }
        return $fb_data;
    }

这是控制器的功能

 public function facebook()
    {
        $fb_data = $this->lib_login->facebook();

        if (isset($fb_data['me'])) {
            echo "<pre>";
             var_dump($fb_data);
            echo "</pre>";
            echo '<a href="' . $fb_data['logoutUrl'] . '">logout</a>';
        } else {
            echo '<a href="' . $fb_data['loginUrl'] . '">Login</a>';
        }
    }

当我使用此代码登录到我的帐户时，注销url和$fb_data数组出现在我的页面上，但当我注销并刷新我的页面时，它仍然是他们的。谁能告诉我这里哪里做错了?