Registered: 13 years ago
You can use php or html (e.g. index.php or index.html). The server recognizes PHP code in your page by your use of tags <?php //code ?> and then interprets (parses) the code and returns an output. Generally you must just be sure that the server is set-up to parse php code in htm pages (if you use htm pages), and it must recognize index.php as an entry page (index.htm and index.html is automatically recognised). XAMPP ensure all of this is in place, so you don't need to worry.
However, when you use *.php pages, Dreamweaver does a far better job in highlighting your PHP code (amongst other things). So I suggest you save it as php.