I know about PHP-GTK and PHP/TK.
On PHP/TK's website, I read:
The Tk extention functions currently only on Unix systems.
On PHP-GTK's website, I read:
That's their last release. In 2008. Even their latest beta version is five and a half years old. And their website is full of dead/"under construction" links/pages.
My conclusion is that both of those projects are abandonware and would be a complete waste of time to spend time learning.
Many things related to PHP and desktop computers in general seems to have died off completely, which makes me extremely uncomfortable.
Please don't start talking about how much "PHP sucks" or "using the right tool for the job". If I've spent 20+ years working with PHP, and made numerous serious attempts to learn other languages only to find that I don't have the time or energy to keep multiple ones in my head, "using the right tool" is not an option. I see no reason why anyone would want to completely re-learn everything just to be able to make a GUI application which isn't web-based. I cannot see any reason why PHP would be unfit for this as it already powers all kinds of "serious work" for me as CLI scripts.
Simply spawning some kind of GUI and interacting with that seems almost trivial (to somebody with the necessary skills) compared to PHP the language itself. Especially if that thing already exists (GTK) and PHP just gets a "thin wrapper" or "language binding" for it. I almost don't even understand why it has to be a "project"; it should be a built-in module of PHP and simply hook into GTK.