As a part of the move to git/github that the PHP project recently made, a reminder was posted that the PHP-GTK project has also migrated.

PHP-GTK is a PHP extension that enables you to write client-side cross-platform GUI applications. This is the first such extension of this kind and one of the goals behind it was to prove that PHP is a capable general-purpose scripting language that is suited for more than just Web applications.

The full source can be found and forked/cloned over on github and is just waiting for you to contribute!