As you correctly state, php-gtk is only for stand-alone applications. It is designed for that purpose only, such as Perl also has a GTK library.
Since when used in web programming, PHP is server side, it is unable to be used for anything interactive that is embedded on the client side. For that to happen, it would have to be like Java, where a plugin was available.
For what you want, AJAX, Flash, or Java applets would suit you well.