There's a quick mention on the PHP-GTK Community website of a tool that developers looking to get into developing PHP-GTK applications might want to check out - Anubis.

Anubis generates PHP-GTK code from your user interface designs and enables you to save and edit them later, including the sources. Unlike UI-only designers (think Glade), you can call the php-gtk interpreter from the IDE just by pushing F9 key once the location of PHP is configured.

Check out the screenshot to get an idea of what it looks like or just head over to Zendrael.com to check it out.