Every linux is slightly different. You have to compile the PHP-GTK that runs on your linux. So it's very difficult for me to help you here.
I have written the following article back in 2006:
Sample Code 66: How to install php gtk2 on linux?
Yes, some of the content might be outdated. But if you follow the same steps (of course replacing the respective libraries with the latest versions), I think you will still be able to compile the PHP-GTK2. The key thing is: the article outlined all the key libraries that you need to make a running PHP-GTK2 for your linux.
You could also do some googling to see if there are more recent articles on how to compile PHP-GTK2.