it depends on the server configuration and php version , file extensions need not reflect php versions which is confusing I know...
If a server has php4.2.3 installed and is configured to pass *.php,*.php4,*.php3 to the PHP interpreter then they will all be passed to php4.2.3 for parsing so in that case there is no difference in functionality between php,php3 or php4 files.
its perfectly possible to have *.yaks, *.gribble etc parsed via PHP (or whatever)
It's possible though this would be the exception rather than the rule....to have 2 versions of PHP running on the same server (one as an SAPI module and 1 as a CGI) , in that case it may be that the extensions offer different functionality, again this is hardly standard.
MVC is the current buzz in web application architectures. It comes from event-driven desktop application design and doesn't fit into web application design very well. But luckily nobody really knows what MVC means, so we can call our presentation layer separation mechanism MVC and move on. (Rasmus Lerdorf)