Alright, first to answer your PHP to HTML question. PHP can be more work to maintain if coded wrong. The way I use it is I setup a single page which is the template of the site and I run all other pages through that single timeplate page. This way when I add a new page, I only have to add the page once. Also, PHP can do form mailers, guest books, members areas, shopping carts, etc. It can be the glue which allows a site to work with a database and display information dynamically.

Now, XHTML vs HTML. The main different is XHTML has a stricter standard which can be easier understood by browsers. For more information go here.
XHTML vs HTML

If you have any more questions feel free to let me know. This is part of what I do for my customers when developing and helping them maintain their site.