One other thing to consider
If your existing site uses html pages, this would be a good time to change all of the extensions to php. You can redirect your old html pages to the new php pages so you don't lose Google juice.
Convert HTML to PHP
The main difference is that the php extension tells your web server that the page may contain php script so it will call the PHP preprocessor. I make all of my pages php so I can always add PHP code latter if I want.