It's been my experience that having your entire site in PHP lets the computer do a lot more of the work and lets you do a lot less (which is how it should be!).
For instance, I worked on a site that, if written in plain HTML, would consist of hundreds of thousands of pages. As it is, it only exists of a few hundred PHP files which change depending on which data they fetch.
Also, converting an HTML file to PHP can be just a matter of changing the file extension from '.html' to '.php'.
