PHP is a widely-used general-purpose scripting language, GTK is a cross-platform widget toolkit for creating graphical user interfaces and PHP-GTK is a set of language bindings for PHP which allow GTK+ GUI applications to be written in PHP.
I have been working on PHP-GTK for some-time, but it seems that PHP-GTK applications crash easily, they are slow and are not according to desktop application expectations. Also I have found a lot of criticism regarding it, like:
if you want to build a desktop application, you can't really do that well in PHP (sorry php-gtk). Then .NET would be a better choice.
PHP-GTK is a real stretch of PHP's intended use. I'd personally stay away.
I agree with the answers there in that PHP is not a language designed for GUI applications and can heartily recommend Python+Qt.
However, as Andrei Zmievski says
I firmly believe that PHP is a better project because of PHP-GTK.
I think PHP V6.0 is coming out any-time soon, GTK+ V3.0 is already there. Is there any work going on for PHP-GTK Version 3.0? Can I expect PHP-GTK V3.0 in 2012?
I have asked this same question over here: