I need to switch my index.html to index.php

By now, I've submitted my site to most major directories in http://www.mydomain.dom/index.html?ref=ref_no form to keep track of my traffic. It's a 3 year old site and there are hundreds of links to it in the form indicated above.

I wouldn't like to have visitors crashing a "your URL is not on this server any more" message.

Can anyone think of a way to handle this?

thank you