Facebook $ helper-> getAccessToken返回null

Question

我已经使用API​​将Facebook登录信息集成到PHP中。它已成功重定向到FACEBOOK凭据页面，并返回到我的主页并检索了代码，但是问题是在获取代码后，我试图获取ACCESSTOKEN，并且它返回null。

下面是我的代码：

require 'fb/Facebook/autoload.php';

function login_via_fb($param = array())
{
    global $outputjson, $gh, $db;
    $outputjson['success'] = 0;

    $fb = new Facebook\Facebook([
          'app_id' => FB_APP_ID,
          'app_secret' => FB_APP_SECRET,
          'default_graph_version' => 'v3.2',
        ]);

    $helper = $fb->getRedirectLoginHelper();
    if(isset($_GET['state'])){
        $_SESSION['FBRLH_state']=$_GET['state'];
    }
    $fb_code = $gh->read('code');
    $logout = $gh->read('logout');
    $permissions = ['email']; // optional

    try {
        if (isset($_SESSION['facebook_access_token'])) {
            $accessToken = $_SESSION['facebook_access_token'];
        } else {
            $accessToken = $helper->getAccessToken(WEB_PANEL_PATH);
        }
        $outputjson['test_Access_token'] = $accessToken;
    } catch (Facebook\Exceptions\facebookResponseException $e) {
       $outputjson['response_error']  = 'Graph returned an error: ' . $e->getMessage();
    } catch (Facebook\Exceptions\FacebookSDKException $e) {
        $outputjson['sdk_Error'] = 'Facebook SDK returned an error: ' . $e->getMessage();
    }
    if (isset($accessToken)) {
        if (isset($_SESSION['facebook_access_token'])) {
            $fb->setDefaultAccessToken($_SESSION['facebook_access_token']);
        } else {

            $outputjson['facebook_access_token'] = (string)$accessToken;
            $oAuth2Client = $fb->getOAuth2Client();

            $longLivedAccessToken = $oAuth2Client->getLongLivedAccessToken($accessToken);

            $outputjson['facebook_access_token'] = (string) $longLivedAccessToken;

            $fb->setDefaultAccessToken($accessToken);
            $_SESSION['facebook_access_token'] = $outputjson['facebook_access_token'];

        }

        if (!empty($fb_code)) {

            try {
                $profile_request = $fb->get('/me?fields=name,first_name,last_name,email');
                $requestPicture = $fb->get('/me/picture?redirect=false'); 
                $picture = $requestPicture->getGraphUser();
                $profile = $profile_request->getGraphUser();
                $fbid = $profile->getProperty('id');
                $first_name = $profile->getProperty('first_name');
                $last_name = $profile->getProperty('last_name');
                $user_name = $profile->getProperty('name');
                $email = $profile->getProperty('email'); 
                $fbpic = "<img src='" . $picture['url'] . "' class='img-rounded'/>";

                $user_exist = $db->select('user_id','tbl_users',' fb_id="' . $fbid . '"');
                $data['username'] = $user_name;
                $data['first_name'] = $first_name;
                $data['last_name'] = $last_name;
                $data['email'] = $email;
                $data['profile_pic'] = $picture['url'];
                $data['user_type'] = 'facebook';
                $data['date_added'] = date("Y-m-d H:i:s");

                if(!empty($user_exist)){
                    $data['date_modified'] = date("Y-m-d H:i:s");
                    $db->update('tbl_users',$data, ' fb_id = "' . $fbid . '"');
                    $data['user_id'] = $user_exist[0]['user_id'];
                }else{
                    $data['date_modified'] = date("Y-m-d H:i:s");
                    $data['fb_id'] = $fbid;
                    $user_id = $db->insert('tbl_users',$data);
                    $data['user_id'] = $user_id;
                }  
                $outputjson['user_data'] = $data;
            } catch (Facebook\Exceptions\FacebookResponseException $e) {
                // When Graph returns an error
                $outputjson['response_error'] = 'Graph returned an error: ' . $e->getMessage();

            } catch (Facebook\Exceptions\FacebookSDKException $e) {

                $outputjson['sdk_Error'] = 'Facebook SDK returned an error: ' . $e->getMessage();
            }
        }


    } else {

        $loginUrl = $helper->getLoginUrl(WEB_PANEL_PATH, $permissions);
        $outputjson['login_url'] = $loginUrl;
        $outputjson['error'] = $helper->getError();
    }
    $outputjson['success'] = '1';
}

如何修复accessToken？ 请帮忙............