facebook iframe app; php sdk getUser() returns valid id on page one but not for any other page

I have a facebook iframe app which correctly logs in and authorizes the app, but getUser() only works on the first page. As soon as a user clicks a link to a new page within the iframe, getUser() returns 0.

What's strange is that this same code works for another app... I do all the clicking I want and getUser() returns a valid ID.

The app that doesn't work: https://apps.facebook.com/celestial_glory/

The one that does (same codebase): https://apps.facebook.com/uprisingstlouis/

Here's the code I am using:

require_once ('fb/facebook.php');



// snip... set $app_id, $secret, and $canvas_page



// first, try normal facebook getUser().  If that works, awesome.



$facebook = new Facebook(array(

  'appId'  => $app_id,

  'secret' => $secret,

));



$signed_request = $_REQUEST['signed_request'];



// Get User ID

$user = $facebook->getUser();

if ($user != '0') return 'fb=' . $user; // works once



// getUser() didn't work.  Try oAuth.  Maybe user needs to log in or

// authorize the game?



$auth_url = 'http://www.facebook.com/dialog/oauth?client_id='

  . $app_id . '&redirect_uri=' . urlencode($canvas_page);



list($encoded_sig, $payload) = explode('.', $signed_request, 2);



$data = json_decode(base64_decode(strtr($payload, '-_', '+/')), true);



if (empty($data["user_id"])) {

  echo '<a target="_top" href="' . $auth_url . '">Login to Facebook</a>';

  exit;

// normally we would auto-redirect, but with a uid of 0, this just auto-redirects

//    echo("<script> top.location.href='" . $auth_url . "'</script>");

} else {

  return 'fb=' . $data['user_id'];

}

any ideas? I have triple-checked app ids and secrets and canvas pages. If those were wrong, I expect no page, not even the first, would work.