That's a good question-- .inc or .inc.php. I'm not actually sure which is best, or if both even work. I'm pretty sure I've seen both, though, so you might want to look that up on google to see where they're referenced. I believe that either way you won't be able to access the page directly, so it should not matter, but just test this on the server.

RE bluewalrus:
I do not believe that you can use a query string in an include statement-- it refers to paths only-- real files. I've tried this before and it hasn't worked. (The exception would be for external links, and that would then probably work as part of the whole URL.) I'm not saying not to try it, but don't be surprised if it doesn't work (and if you DO get it to work, post back with details).
The easy way around this is to set another variable or what is more often seen in software is setting a constant: define('VALID',1); (in the main page), then if (is_defined('VALID')) { .... } in the included page. Or just a variable in the same way.