If you’ve used server-side includes, you know that you usually have to rename your web pages from “something.html” to “something.shtml” in order to get the server to “do” the includes. The extension “shtml” lets the server know that it has more work to do on the web page before the reader gets to see it. Similarly, your “PHP” web pages usually need to end in the extension “php”. This lets the server know that it needs to hand this page off to the “php” module before letting the reader see the web page.
Some servers will be set up to require an extension of “.php3” instead of “.php”. The choice of extension is completely up to whoever sets up your server. While it will usually be “php”, it can be anything. Contact your system administrators to be sure.