If you are talking about using PHP as a language to program graphical applications, I do believe they have developed a GTK package for the GNOME desktop under linux that will allow you to do just that. Last I heard about it was a couple of months ago it was just getting finished for initial release, but I don't know where things stand as of late. Check out www.gnome.org or something. Maybe sourceforge.net would have some info, too.
