I think the PHP Gtk binding are still a compile from source mission on ubuntu, there was not a package available when I needed it a few weeks ago. Plus if you leave the module in your default php config, it breaks php (as in won't run even non Gtk scripts) whenever it it run from somewhere without a window manager available (ssh, cron, etc).
All in all a pain in the ass.
I'm about to give up installing the requirements for a GUI on CentOS 5.3 trying to get php-gtk installed is quite the venture.
"Love may fail, but courtesy will previal." -- A Kurt Vonnegut fan