The Developer Tutorial site has an introduction to PHP-GTK2 posted - an overview of what the new version (version 2) is and how you can install and use it to develop desktop applications.

PHP-GTK 2 is out, and with it, a fantastic means for PHP developers to build useful, reliable cross-platform desktop applications. In this tutorial, I'll show you how to get up and running with PHP-GTK in no time and build your first desktop application.

They look at qhat it is, getting started with it and making your first application (a simple little "Hello World" output box). They also point out some of the demos you can grab as examples and show you how to make something a little more powerful - a window with an input form that generates an md5 hash.