On the Zend Developer Zone, ahamedjais has written up a tutorial that talks to the reader about the creation of an example PHP desktop application via PHP-GTK.
We have been creating web-applications using PHP since it came into existence, now we can also develop Desktop or Stand alone Applications with PHP-GTK. [...] Thanks to PHP-GTK, we can now build cross platform windowed applications with PHP as well.
He starts off by describing what PHP-GTK is and the essentials (like where to get it and how to test to see if your installation is working with some sample code). He also includes links to more resources about PHP-GTK including a PHP-GTK cookbook and a page describing all of the base GTK classes.