使用Facebook登录获取电子邮件地址

我正在尝试从facebook登录获取电子邮件地址。

请参阅代码

    <?php
    // added in v4.0.0
    require_once 'autoload.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'Entities'AccessToken;
    use Facebook'HttpClients'FacebookCurlHttpClient;
    use Facebook'HttpClients'FacebookHttpable;
    // init app with app id and secret
    FacebookSession::setDefaultApplication( $fb_app,$fb_key );
    // login helper with redirect_uri
      $helper = new FacebookRedirectLoginHelper('http://my-site.com/fbconfig.php' );
    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();
            $fbid = $graphObject->getProperty('id');              // To Get Facebook ID
            $fbfullname = $graphObject->getProperty('first_name');
            $femail = $graphObject->getProperty('email');    // To Get Facebook email ID
        /* ---- Session Variables -----*/

    } else {
      $loginUrl = $helper->getLoginUrl();
     header("Location: index.html");
    }
?>

一切似乎都很好，除了电子邮件。

我使用$femail = $graphObject->getProperty('email');行来获取用户的电子邮件地址。

谁能告诉我如何解决这个问题?