Facebook api getLoginUrl是空的

Question

我正在尝试使用Facebook登录按钮构建一个网站，但是当您点击登录按钮时，它会将您带到我的login.php页面，该页面只是空的。据我所知，getLoginUrl只是空的。这是我的代码：

FacebookSession::setDefaultApplication(FB_APP_ID, FB_APP_SECRET);
$helper = new FacebookRedirectLoginHelper(FB_REDIRECT_URI);

if(isset($_GET['type']) && $_GET['type'] == 'facebook' ){

    $fb_url = $helper->getLoginUrl(array('email'));
    header('Location: ' . $fb_url);
}

$session = $helper->getSessionFromRedirect();

if(isset($_SESSION['token'])){
    $session = new FacebookSession($_SESSION['token']);
    try{
        $session->validate(FB_APP_ID, FB_APP_SECRET);
    }catch(FacebookAuthorizationException $e){
        echo $e->getMessage();
    }
}

$data = array();

if(isset($session)){
    $_SESSION['token'] = $session->getToken();
    $request = new FacebookRequest($session, 'GET', '/me');
    $response = $request->execute();
    $graph = $response->getGraphObject(GraphUser::className());

    $data = $graph->asArray();
    $id = $graph->getId();
    $image = "https://graph.facebook.com/".$id."/picture?width=100";
    $data['image'] = $image;
    if($user_obj->fb_login($data)){header('Location: home.php');}
    else{header('Location: index.php');}
}

有谁知道我的代码有什么问题？