Facebook PHP SDK getSession返回NULL

Question

use Facebook\FacebookSession;
use Facebook\FacebookCanvasLoginHelper;
use Facebook\FacebookRedirectLoginHelper;
use Facebook\FacebookRequest;
use Facebook\FacebookResponse;
use Facebook\Entities\AccessToken;

FacebookSession::setDefaultApplication($client_id,$client_secret);

$helper = new FacebookCanvasLoginHelper();

try {
    $session = $helper->getSession();
} catch (FacebookRequestException $ex) {
    echo $ex->getMessage();
} catch (\Exception $ex) {
    echo $ex->getMessage();
}

    var_dump($session); exit;

为什么这段代码会在index.php中返回正常会话，但在其他页面上返回NULL？