PHP-GTK is only for desktop applications I believe. You mentioned "site", so here is my suggestion based on previous experience.
Build the site in FireFox and deploy it, including Apache, PHP and MySQL. All you really need to do is make sure to style the elements big enough to use with a touch screen and the only licensing issue is with MySQL. You can deploy 3.x without a license, but newer versions require one for commercial deployment. You can program your KIOSK to look at a specified IP to act as the data server is you don't want local databases as well.
If you PM me I will shoot you a link to a site with example screen shots ... the company I worked for.
edit
You can use JavaScript to hide toolbars as well, to give a real "program" feel.