Facebook API - how to get page ID using Facebook API?

With Facebook API, within a custom app added to a fanpage (tab app), how do I get the fanpage ID?

Here's what I'm trying to achieve:

• user goes to the full-width canvas app, he authorizes himself there and allows the app to add a tab to one of the fanpages he's administrating
• every Facebook user going to that fanpage, sees the tab and the tab shows content specific to that fanpage (that's why I need to know the fanpage ID)
• if fanpage administrator wants to change something, he goes to the full-width canvas and logs in to my service (here's where I store what he wants t开发者_高级运维o show in his Fanpage)

From a parsed signed request (to an tab iFrame) you can get the page id - the contents would be similar to the following (output of PHP print_r):

stdClass Object
(
    [algorithm] => HMAC-SHA256
    [issued_at] => xxxxxxxxxx
    [page] => stdClass Object
        (
            [id] => FAN_PAGE_ID // target page id
            [liked] => 1        // is the user a fan
            [admin] =>          // is the user an admin
        )

    [user] => stdClass Object
        (
            [country] => ie
            [locale] => en_GB
            [age] => stdClass Object
                (
                    [min] => 21
                )

        )

)

For more information see: http://developers.facebook.com/docs/authentication/signed_request/