If your httpd is configured to associate .inc file extensions with PHP, you can simply make the .inc file as you would any other .php script. Then add the code that you wish to have included in the file.
A good idea though, would be to just use the normal .php extension for the include files, in case you change web hosts, or have a situation where .inc isn't being parsed by PHP, you won't have to worry about the source being available by anyone grabbing the include file directly.
Other things to consider regarding security measures to implement with the include files... As always, verify all incoming arguments, and if possible limit them to none or just the bare neccessities. Also, read up on the include_once() and require_once() functions for added precautionary measures.
Good luck.
- tre
<a href="http://piclabs.com">PicLabs.com</a>