Access token gets lost after visiting another page - facebook

Question

When I visit index.php, everything works well and I see the acces_token appear in the session variable.
However, when I click on a link to go to a new page, and I click (on the new page) on a link that brings me back to index.php, the access_token is changed to a much shorter token, starting with my appId.
(edit: I just discovered that this shorter token is this: appid|appsecret)
I've tried linking to an exact copy of index.php, and the same thing happens. It's like there isn't a new facebook instance created, or something... Why isn't a new access_token created?
Below is my code. You'll notice that I don't log the user in on my app. That's because this part is done on another page, via the Javascript SDK.
I've been looking for hours... thanks in advance for your advice.
index.php
<?php
include "config.php";
require "ok.php";
?>
<br/><br/>
Link.
<br/><br/>
<?php
$user = $facebook->getUser();
$userInfo = $facebook->api("/$user");
?>
<?php d($user); ?>
<strong>PHP Session</strong>
<?php d($_SESSION); ?>
?>
ok.php
<?php
$fbconfig['appid' ] = "###";
$fbconfig['secret'] = "###";
$user = null;
try{
include_once "facebook.php";
}
catch(Exception $o){
echo '<pre>';
print_r($o);
echo '</pre>';
}
$facebook = new Facebook(array(
'appId' => $fbconfig['appid'],
'secret' => $fbconfig['secret'],
'cookie' => true,
));
function d($d){
echo '<pre>';
print_r($d);
echo '</pre>';
}
?>
config.php
<?php
session_start();
//We log to the DataBase
mysql_connect('...', '...', '...');
mysql_select_db('...');
?>