I hope this isn't a dumb question... It's really a curiousity thing so don't ask why I'm asking, I'm just trying to learn. :)
I have a php website. My "header" is in a separate file so I use an includes statement on all my regular pages. If I put header.php in the robots.txt file, will that either 1 - cause an error to the robots or 2 - disallow the header on all my regular pages?
The inclusion of that header is done at a different level; for practical purposes, the header is part of the page, and is not requested separately (by a browser or by a robot). For that reason, disallowing it in robots.txt won't have any effect.