Phoronix: PHP Slowly Progresses For GLib, Cairo, Pango
While the PHP-GTK project has basically been dead for years, to provide GTK2 bindings to the PHP programming language, there have been several other projects worked on in more recent times to advance the GNOME support for the PHP programming language...
There are also other bindings for the PHP language to other crossplatform GUI libraries like QT http://php-qt.berlios.de/ that seems pretty much unmaintained and the wxWidgets one that seems to have progressed to a usable level: http://wxphp.org/
Not yet, because gtk in browsers is still experimental. For developing applications via browsers, not websites, this might be feasible.
In which case server-side is irrelevant, because the app is in the browser. The fact of the matter is that there's pretty much no value in having PHP support these things. Yes, you can write regular programs in PHP and run them from the command line or wherever, but there are far better languages to do that than PHP (I mean, there are far better languages for server-side code than PHP, but that's an argument for a different day). Just because it's technically possible to use PHP to do this, doesn't mean it's a good idea or a good use of developer resources.