Well usually its not a very good idea to let PHP parse .html/.htm files but its possible. It's in the apache config. Try to find the line:
AddType application/x-httpd-php .php .phtml .php3
..and just add the .htm and .html with the rest.
If you run many other sites with that too, you could also just add the type in .htaccess file too to only reflect that particular directory and its subdirectories.
But as I said earlier, it IS better to keep static files static. Remember that when you start a static html page, it loads with PHP consuming more memory than it has to.