A quote from this web page plus there is some good reading here including the comments at the bottom of the page:
PHP-GTK is an extension that allows you to create GUI standalone, cross-platform applications. This extension is not used for the web in any way, but for client-side applications.
I would suggest downloading a version of PHP just to install PHP-GTK on and not to use the PHP version in XAMPP. I doubt that the XAMPP version of php would be compatible with PHP-GTK.
From what I understand of it all GTK+ is a programming language for desktop use rather than for use with a web server. Much like Java, VB etc. etc.
Is it not possible for you to download and install a binary for a particular version of PHP you download for just that purpose from:
and then install it yourself?
Some more interesting reading: