Facebook App Domain dissapears

I've added a comment box to my site. I'm using an app_id. I've added my domain to "App Domain" in the settings section for the app. I save it. I then come back and the "App Domain" has disappeared. It's like it's not being saved.

One Tomcat instance for two domains and two webapps

How can I configure Tomcat (in standalone mode, that is without Apache [*]) so that I can deploy it on one server and have it serve two different webapps, depending on the domain name requested?

How do I select only the root domain from $_SERVER["HTTP_REFERER"];?

I want to have a 404 page that can detect if a user has come to that page either via my site, via my shortened URL or via another site, and I am making it using PHP. I am slowly getting to grips with PHP and this may well be a simple question, but I am quite tired to be honest and have no caffeine in my system, and I am wanting to tie up any loose ends in my portfolio as soon as possible.