title: FB CanvasLoginHelper object is missing information for some users causing a NULL session
body: <p>FB CanvasLoginHelper object is missing information for some users causing a NULL session for those users.  </p>
  
  <p>Does anyone have any ideas on how to fix this? Any ideas why the $helper would fail for some users but work for others or why it would fail temporarily for some users?</p>
  
  <p>Using PHP SDK 5.0.0</p>
  
  <p>Code has been in use for about 2 years, running properly for all users without any significant changes over that period of time. It suddenly stopped working a few nights ago. Some users weren't affected at all, some were affected for a short time (about 16 hours) and others are still affected. </p>
  
  <p>No errors are generated.</p>
  
  <p>See code below. </p>
  
  <p>It affects all browsers. Tried having the users use multiple bronwers, clearing all caches and doing a device restart. Nothing fixed the problem.</p>
  
  <p>I did var_dumps of the objects and compared the results for users who are working vs those that are failing. Here are the differences in var_dump output.</p>
  
  <pre><code>$helper : 
      ["rawSignedRequest"] is much shorter (188 characters vs 546)
      no [expires] property
      no [oauth] property
      no [user_id] property
  
  $signed_request :
      ["rawSignedRequest"] is shorter
     no [oauth_token] property
     no [user_id] property
  
  $session :
     returns NULL
  </code></pre>
  
  <p>I've searched for similar issues without any luck. Most identify a missing session_start() after the opening 
  
  <hr>
  
  <pre><code>&lt;?php
  session_start(); 
  require('../Session/init.php');
  require_once '../Facebook/autoload.php';
  
  FacebookSession::setDefaultApplication( 'myAppId','mySecret'  );
  $helper = new FacebookCanvasLoginHelper();
  var_dump($helper);
  
  try {
    $session = $helper-&gt;getSession();
    $signedRequest = $helper-&gt;getSignedRequest();
    var_dump($signedRequest);
    } catch(FacebookRequestException $ex) {
    logError('FBIndex - New Session', $ex-&gt;getCode(), $ex-&gt;getMessage());
    echo "&lt;div&gt;Error connecting with FB. Please try again later.&lt;/div&gt;";
    exit();
    } catch(\Exception $ex) {
    logError('FBIndex - New Session', $ex-&gt;getCode(), $ex-&gt;getMessage());
    echo "&lt;div&gt;Error connecting with FB. Please try again later.&lt;/div&gt;";
    exit();
    }
  
  var_dump($session);
   if ($session) {
  etc...
  </code></pre>