Facebook SDK登录$ helper-＆gt; getSessionFromRedirect（）始终为NULL

Question

我还在学习这个SDK登录。

我使用here中的代码（解决方案）。

已经尝试过不同的来源，但仍然遇到同样的问题。

PHP：     

// added in v4.0.0
require_once( 'Facebook/FacebookSession.php' );
require_once( 'Facebook/FacebookRedirectLoginHelper.php' );
require_once( 'Facebook/FacebookRequest.php' );
require_once( 'Facebook/FacebookResponse.php' );
require_once( 'Facebook/FacebookSDKException.php' );
require_once( 'Facebook/FacebookRequestException.php' );
require_once( 'Facebook/FacebookOtherException.php' );
require_once( 'Facebook/FacebookAuthorizationException.php' );
require_once( 'Facebook/GraphObject.php' );
require_once( 'Facebook/GraphSessionInfo.php' );

// added in v4.0.5
use Facebook\FacebookHttpable;
use Facebook\FacebookCurl;
use Facebook\FacebookCurlHttpClient;

// added in v4.0.0
use Facebook\FacebookSession;
use Facebook\FacebookRedirectLoginHelper;
use Facebook\FacebookRequest;
use Facebook\FacebookResponse;
use Facebook\FacebookSDKException;
use Facebook\FacebookRequestException;
use Facebook\FacebookOtherException;
use Facebook\FacebookAuthorizationException;
use Facebook\GraphObject;
use Facebook\GraphSessionInfo;

// start session
session_start();

// init app with app id and secret
FacebookSession::setDefaultApplication( 'xxxxxxxxxxxxxxxxxx','xxxxxxxxxxxxxxxxx' );

// login helper with redirect_uri
$helper = new FacebookRedirectLoginHelper( 'http://os.topclass.com/' );

// 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;
  }

} else {
  // no session exists

  try {
    $session = $helper->getSessionFromRedirect();
  } catch( FacebookRequestException $ex ) {
    // When Facebook returns an error
  } catch( Exception $ex ) {
    // When validation fails or other local issues
    echo $ex->message;
  }

}

// see if we have a session
if ( isset( $session ) ) {
    var_dump($session);exit;
  // 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' );
  $response = $request->execute();
  // get response
  $graphObject = $response->getGraphObject()->asArray();

  // print profile data
  echo '<pre>' . print_r( $graphObject, 1 ) . '</pre>';

  // print logout url using session and redirect_uri (logout.php page should destroy the session)
  echo '<a href="' . $helper->getLogoutUrl( $session, 'http://os.topclass.com/logout/' ) . '">Logout</a>';

} else {
  // show login url
  echo '<a href="' . $helper->getLoginUrl( array( 'email', 'user_friends' ) ) . '">Login</a>';
  var_dump($session);exit;
}

问题： 会话始终为NULL我已尝试登录fb和var_dump($session);，但结果为NULL

我确定app id和secret已经正确了。 在开发人员fb

App域名：os.topclass.com

网站UrL：http://os.topclass.com/

我错在哪里？请帮忙。

更新问题：

 $session = $helper->getSessionFromRedirect(); --> this alwys return NULL