A link I may come to regret showing Deane: DevX has an article on PHP-GTK, which provides PHP bindings to the GIMP Tool Kit, the foundation of the GNOME desktop environment.
Now, PHP has begun to carve a space for itself in the standalone application arena as well with PHP-GTK. As its name indicates, PHP-GTK combines PHP with the GIMP Tool Kit (GTK), an object-oriented framework for developing graphical user interfaces (GUIs). GIMP (GNU Image Manipulation Program), which was developed using X11 on Linux platforms, basically enables the same PHP-GTK code to run on Linux, MS Windows, BeOS, and Mac 0SX without any changes.
So while PHP requires a server — often Apache — to run, you can use PHP-GTK to create standalone GUIs that run on Windows, Linux, or Mac desktops, independently of a server.