xHTML is HTML that conforms to valid XML syntax, strictly. It's really up to you. I tend to use xHTML because I like doing DOM tricks and HTML validators don't do the best job at checking for conformance errors that will crash the crap out of the XML validator when I try to build the DOM.
Here's a link to briefly outline the differences:
http://www.w3schools.com/xhtml/xhtml_html.aspUsing CSS is almost mandatory with xHTML since alot of the tags used to control styling (like ALIGN) are deprecated, forcing use of css stylesheets or style attributes. As far as CSS versions, version n+1 just expands on version n, but many browsers don't support the latest/greatest CSS, so keeping the CSS as basic as possible is the approach I would take.
The advantage of using PHP is that it is server-side and dynamic, so you get the dynamic pages without the incompatibilities introduced with doing things clientside, with say, javascript to get dynamic data.
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