In the past I have used mod_rewrite to redirect .html file request to the equivalent .php file names. For example:

RewriteRule (.+)\.html? $1.php

The above code would redirect any page name page.htm or page.html to page.php.

But what if you had .html or .htm file that you actually wanted to run through the php processor? Try this in your .htaccess file:

AddType application/x-httpd-php .php .htm .html