The question doesn't quite make sense. Is this an external WebAuth page or an internal WebAuth page? For internal, you're going to have to stick with basic HTML/CSS. If it's an external page, then you can use whatever you want, so long as you include the required data provided by Cisco in their WebAuthBundle file (ie. form/actions/script items). It would make no sense that you cannot use PHP or any other server side technologies on an "external" page because the WLC would have nothing to do with it.
