title: $facebook -> getUser() returns wrong value?
body: <p>The question is the same as title. I'm using the latest php-sdk (v3.1.1) to operate server-side authentication flow.</p>
  
  <p>I have 2 tabs in Chrome, one is my Facebook page and the other is php test page. These 2 problems happen many times:</p>
  
  <ul>
  <li><code>$facebook-&gt;getUser()</code> still returns 0 even when I logged in.</li>
  <li><code>$facebook-&gt;getUser()</code> still returns an ID even when I logged out.</li>
  </ul>
  
  <p>I have to do a work-around of this: try initiating a graph API request with provided access_token, and check if <code>$response-&gt;error-&gt;type == "OAuthException"</code> to ensure there's an active session or not.</p>
  
  <p>Is there any way to use <code>$facebook-&gt;getUser()</code> "stably"? I've searched a lot through SO but not found best answer for php-sdk 3.1.1 yet.</p>
  
  <p>Highly appreciate any helps. Thanks.</p>