as others have said, it doesn't matter what your pages are written in. all clients should care about is the Content-Type of the results.
that said, it is highly recommended that you make use of technology like mod_rewrite to eliminate filename extensions from urls. this will help you when, a few years down the line, you decide to rewrite your site in a different language. if you have to change urls, you'll break other sites' links (and lose PageRank with google).
search engines are also usually a little skittish about indexing content that looks dynamically generated. this is based not on filename extensions but by parameters in the url (ie if there is a '?' with anything after it). eg, google will follow links to a page with parameters, but won't follow links from that page to other dynamic pages. this is to avoid getting the spider caught in an infinite loop (imagine if a site appended a random number to every url.) again, using something like mod_rewrite to change your urls from 'product.pl?product_id=1234' to something more like '/products/1234/' will make it much easier to index.
