extention name

(1) file.php
(2) file.php3


What is the difference between the extention names?

In version 3 of PHP, the extension was .php3 - now it's just .php

There is no difference between
.php, .php3, .php4, .pthml, .blah
as long as your .htaccess is set to parse files with those extemtions as php. In your .htaccess add:
AddType application/x-httpd-php .php, .php3, .icn, .blah, .codingforums

or whatever
:)