For a client I have been redesigning my website and I am switching it from HTML to php. Currently, their PR is of level PR3 on Google right now. The part where I was about to change their index.html to an index.php was confusing me. If I changed their index from HTML to php, is there a possibility that their PR would be affected in some way? I am not bothered about the rest of the site as the only PR they have is on the index.

Can anyone help me out here?