.html Versus .php for SEO
I was asked the other day if search engines favor a .html file extension versus a .php file extension? The answer I gave the client was no, it makes no difference, search engines will index pages with both file extensions the same, but there are some things that can make a difference. If you are using a GET query string on the .php page then the Google Webmaster Guidelines suggests that the parameters should be kept short and limit the number. I suggest no more than three, but some people say up to five is fine. It is also suggested to keep session ids out of the URL.
If people are serious about not using a .php file extension, there are other solutions. If you are just using php to template your website, you can just process .html as .php.