php-gtk2 (php5/gtk2) is underway , so not only can your web-based applications be x-platform but any supporting GUI's can be as well (yes you could use python/tcl etc but why would you with a stable php-gtk?)
Because PHP GTK hasn't gone very far in years I'm pretty disappointed in that. That looked really promising, now I'm not sure where it's going...
its going places , Andrei is working on GTK2 now , it will not be ready tomorrow but it will happen , we even had some commits/bug-fixes to the php-gtk 1 codebase , the documentation is still a long way from complete but its getting better daily.
I was surprised to see in a recent mailing list thread the amount of uses people already have for PHP-GTK even with the existing code within commercial environments .
I personally cant see how people can not get excited about php-gtk though many don't (including Rasmus I hear) , its puuurrrfect for x-platform client-server integration , & since its all PHP you can use the same codebase , use serialized objects ... use all the good stuff that PHP has wherever/however you need it (shell/SAPI/gui/daemon)