CSS and PHP can support any DTD. As of now I'm going with the HTML 5 DTD. As long as a page was written to be rendered in standards mode and not quirks mode, HTML 5 is completely backwards compatible with either XHTML or HTML of almost any flavor.
When it comes to XHTML, we have a great reference thread that gets into the issues pretty well Why most of us should NOT use XHTML [webmasterworld.com]
XHTML was one of those times where "everyone says" was not the best guidance to follow.