Whether you use .php, .asp, .aspx, .shtml etc., all pages are still html. Even if you do not have any php code in the page, you can still use the .php extension. So instead of index.html, use index.php. This will save you a lot of time in the future plus, more importantly, you won't have to use any 301 redirects when it becomes necessary to change file extensions. Your pages will already be properly index.