According to this new post on the main PHP-GTK project's website, they've officially made the move over to github:

Although the main PHP-GTK site at http://gtk.php.net/ does not (yet ?) make mention of it, development of PHP-GTK is still active, but is using a more recent infrastructure, having moved to Github like many other projects.

You can find this new repository at https://github.com/php/php-gtk-src. The PHP-GTK project is an extension for the PHP programming language that implements language bindings for GTK+. It provides an object-oriented interface to GTK+ classes and functions and greatly simplifies writing client-side cross-platform GUI applications.