Does anyone know if having index.html and index.php under one domain name will harm page rank and or serps ? (mods, please move if not in right place)
No, but it will complicate things. (And I think it will go to index.php rather than the .html version) It is highly unrecommended. Hope I've helped.
I agree with Geoffrey. The index.php will be sent to the browser instead of index.html. PHP is severside scripting not client side. Your best bet is to just use index.php, since php file display html code just fine. if you want to keep the index.html on your site, maybe you should rename it, and if you want to use wat is in the index.html file, you can with php using the include function.