You can include files of any extension, although keeping it as a *.php extension is a good idea for security purposes. If you don't want people to have access to the source php code in the include file, saving it with a php extension will mean that it will be parsed when it is retrieved by the user. This is important when the include is perhaps connecting to a database and will contain login information, or you just don't want people stealing your code.
To include a file simply use the following:
<?
include("the_inc.php");
?>
Remember that includes don't use URLs to locate the file instead they use paths, for example:
<?
//this in NOT correct
include("http://www.foo.com/the_inc.php");
?>
<?
//this in correct
include("/www/vhtdocs/foo/the_inc.php");
?>
