The project's website (http://gtk.php.net/) looks like it's been neglected for over a year. There are some bad links in the resources section, and the last news update was the 28th of May, 2009. The latest version doesn't support php 5.3 (it was released in 2008).
How will the Phoronix Test Suite's GUI frontend cope if/when PHP-GTK gets too obsolete?