There is an installer for PHP, GTK, and PHP-GTK. http://pigii.sourceforge.net/
It worked for me on a windows xp machine without any fuss. The glade user interface came up and seemed to run alright, I didn't save any projects or test it thoroughly, so don't quote me on that. The PHP-GTK demo worked.
The part I have issues with is getting the gtkhtml extension to work with the 2.0.1 release of PHP-GTK. I've seen things around the net, but none of them really work. I suppose you can build GUI applications with PHP-GTK if you're happy with the extensions that work. I don't know. Even Gnome is talking about dropping GTK, and the PHP-GTK website is suffering link rot. I checked out a copy of the PHP-GTK source from subversion, thinking "well, maybe I could compile the php-gtk, php extensions, and gtk extensions. . . myself" My plan was foiled though. It seems the snaps server is down, broken, whatever, and I can't get source from the same version of php as was used to build php-gtk.
So basically I'm looking at
1. GTK's future is on the "should we toss it list" from it's own team.
2. PHP-GTK is what it is, with a sparse manual, and no clear path to upgrade or develop.
3. PHP snapshots of the extensions and code available at the time of PHP-GTK's release, are currently (permanentely??) unavailable.
I saw a little bit about an extension for PHP and QT, but that seems to have gone nowhere.
And then there's COM. . . :( COM is really cool for my windows machine but, word on the streets is that it's useless on a linux box. :(
I wish we could make simple desktop apps with PHP 5.3.6 and some current widget framework.
It is what it is I guess.