Looking to make a simple PHP include...

[Crash_SCM]

I want to include a PHP file ever 5 posts. I've been looking through the templates and reading how to do certain stuff on this board. What I believe I need to do is create a hook(?) and then have the code include the php. Is this correct?

To be more specific, I'd like to place the file after the 5th and 10th posts in a thread on each page of the thread, (so obviously it would be after post 5, 10, 20, 25, 35, 40, etc..)

Anyone have some insight for me? A path to get me started on? I'm not new to programming, but I'm new to vB mods.

Thanks,
Matt