On the PHP-GTK blog today there's a new post looking at getting the latest versions of PHP (5.3) and PHP-GTK up and running on a Ubuntu-based system.
At the moment there are some minor problems when trying to get a working installation of PHP with GTK running on Ubuntu based systems. Here is a quick walk through of how you can get this all set up with the trunks from GTK and Cairo and the official package for php-cli in 10 easy steps. This installation was originally done on Linux Mint 9 but should work with most Ubuntu based Linux distributions.
There's ten steps in the process including fetching the packages you'll need with an apt-get call, checking out the latest cairo and php-gtk and the commands to patch and build the source.