Is the requirement that when you hit www.somedomain.com/ the index.html content shows by default, but you want the index.php stuff to show? If that is the case, you can set the apache "DirectoryIndex" directive to include index.php
If you want users to hit index.html, and get PHP interpreted stuff inside the page, you can't really do that. Technically you could tell PHP to interpret files that end in .html, but I wouldn't really recommend doing that.
As Massimo said, if you want users to specificly hit index.html and end up witha php processed page, RewriteRules would be required for that.