I saw this article a few months ago and promptly lost it again; the wonders of Google.
The important part of the article isn’t actually the PHP bit (I know very little PHP), but the fact that it shows you can deliver an enhanced browsing experience to people using Gecko-based browsers. The example they give is found at http://www.phppatterns.com/xul/ – if you try and view this file in a browser that doesn’t support XUL you’ll probably be asked to download the file. If, on the other hand, you’re using Mozilla, Firebird, Camino or one of the others you’ll see a some menus across the top of the screen (there’s a screenshot in the article).
In fact, if you can redirect requests based on the user-agent you can give your website proper tree widgets, tables, radio buttons, checks, the works. Your website becomes a mini web application.
Of course this isn’t necessarily an ideal situation, and at first glance has at least a few drawbacks, the most obvious of which is that you lose bookmark-ability for web pages. There’s possibly a workaround, but it’s too late at night to think of one now.