Is it hard to redesign a PHP dynamic site? Some of the pages are generated from database. What kind of things should I look out for?
Any advice is appreciated. Thank you.

It depends on how it was written. I mean engine, how does it works. If it obtain info from a db, and then display it using a template - it should be quite easy, just redesign the html template And if it obtain info and display it immideatly - it will be a little bit harder -you have to change a code that display info...

Be sure to create as little as possible temlate files.

If you can get away with just doing Heder.php and footer.php do it.
When pulling from a database try to use these also in includes. This way you can edit its search patern if needed.