Getting current user ID in page
===

I'm trying to find the ID of the user visiting my page tab (without having to make him authentify before using the page)

I've tried several methods:

    $requete_fb = $facebook->api('/me');

    $me = $facebook->api('me', array('fields' => 'username'));

    $request = $facebook->getSignedRequest();

    $uid = $facebook->getUser();

But all of them did not work (getUser returns me 0, the others PhP errors.)

So how can I do that? I'm not trying to break the TOS, I'm trying to fetch basic information (the locale).

Any other way to do it?