PHP-GTK » "Too often PHP is thought of as only an HTML-embedded web scripting language. However, it is also a very full-featured general purpose language that can be used for much more. One of the goals behind this project was to prove that PHP can be used to write client-side GUI applications." Can I just add "O yeah!" to that? This looks good. And did you see these screenshots? This rocks.
Update: you can also find a presentation on the subject.