刷新网页时Facebook会话消失

我是新来的facebook登录SDK的PHP，我正试图创建一个"登录与facebook"的功能在我的网站。

我正在使用这个代码，它工作得很好:

session_start();
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/FacebookAuthorizationException.php' );
require_once( 'Facebook/GraphObject.php' );
require_once( 'Facebook/HttpClients/FacebookHttpable.php' );
require_once( 'Facebook/HttpClients/FacebookCurl.php' );
require_once( 'Facebook/HttpClients/FacebookCurlHttpClient.php' );
use Facebook'FacebookSession;
use Facebook'FacebookRedirectLoginHelper;
use Facebook'FacebookRequest;
use Facebook'FacebookResponse;
use Facebook'FacebookSDKException;
use Facebook'FacebookRequestException;
use Facebook'FacebookAuthorizationException;
use Facebook'GraphObject;
use Facebook'FacebookCurl;
use Facebook'FacebookCurlHttpClient;
use Facebook'FacebookHttpable;
FacebookSession::setDefaultApplication('app-id', 'app-secret');
// login helper with redirect_uri
$helper = new FacebookRedirectLoginHelper( 'app-redirect-link' );
try {
  $session = $helper->getSessionFromRedirect();
} catch( FacebookRequestException $ex ) {
  // When Facebook returns an error
} catch( Exception $ex ) {
  // When validation fails or other local issues
}
// see if we have a session
if ( isset( $session ) ) {
  // graph api request for user data
  $request = new FacebookRequest( $session, 'GET', '/me' );
  $response = $request->execute();
  // get response
  $graphObject = $response->getGraphObject();
  // print data
  echo  print_r( $graphObject, 1 );
} else {
  // show login url
  echo '<a href="' . $helper->getLoginUrl() . '">Login</a>';
}

如果我按"登录"，我得到我的详细信息和我需要的一切，但只要我刷新页面-"登录"链接再次出现，好像会话不再存在了。