As far as I know, PHP-GTK seems to be dead or at least very inactive. Not that he ever had a very active community ...
There are also several other PHP GUI projects, I personally would not use them, for the simple reason that IMHO PHP lacks functions for writing serious applications with a graphical interface, the biggest is probably a lack of threads or any other serious features of multiple processing if for that matter ...
There is pcntl_fork() that works (used several times), but it has a major flaw that it doesn't work on Windows. It can also be argued that using fork() worse than using threads, but this is another discussion.
In particular, with the graphical interface, you probably want to get some kind of thread support, let's say your graphical interface does some operation that takes 20 seconds (copy files, upload something, etc.), you usually donβt want to so that the user interface freezes during this operation, and you can also run multiple operations at the same time. I'm not sure how PHP-GTK solves these problems if they solve them at all, but the lack of real built-in language support will still cripple you.
There are other disadvantages of PHP, but they are more general, although you are likely to come across them earlier on desktop applications than on web applications (for example, error handling, OS portability, proper support for UTF-8, etc.) .
There are many other programming languages ββthat are much better suited for this particular job, and most of them have the advantage of having (a lot more) community than PHP-GTK, which means more documents, for example, more people apply for help etc.
C ++ can be a good choice, Python, Perl, Ruby, C #, etc. etc. can also be considered. Wikipedia has an extensive list: http://en.wikipedia.org/wiki/Comparison_of_programming_languages
My advice: choose the one that looks βOKβ and meets your needs, and start hacking. If you don't like it, try something else .;)