One (probably not ideal) way to get round this might be to have a file in your root directory called index.html with no content other than to have a META redirect in the <head></head> area with a refresh delay of 0.
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Refresh" content="0;URL=index.php" />
This assumes that .html is higher than .php in the default files hierarchy on your server - (it usually is).
Might be worth a try. You could also - while you're at it - add all the Meta Description and Keywords into the head of the html document too.