It sounds like a webserver configuration issue to me. The web server is saying that it doesn't know how to handle the request.
"The 500 Internal Server Error is a very general HTTP status code that means something has gone wrong on the web site's server but the server could not be more specific on what the exact problem is."
Can you confirm that php is working? (maybe by creating a test.php file with the contents of <?php phpinfo(); ?> and browse to it?