File extensions can be anything you like ... php, php3, phtml, or for that matter, kwisatzhaderach. The phtml extension was more common in the days of php/fi (also known as php 2). Recent Linux distributions of Apache+PHP tend to be configured for php and php3 extensions.
The important thing is that you configure your Web server to process the filename extension that you choose as PHP code. That might even be html, if you want all of your html pages to be run through the PHP engine (at the cost of some processing time).
With Apache, to define PHP types, a line like this goes in the conf file:
AddType application/x-httpd-php .php3 .phtml .php
To define a type that will let you display PHP source code in pretty colors, do this:
AddType application/x-httpd-php-source .phps
