For a new site which I just got indexed, I have noticed G has indexed php includes... WTF? I am totally sure I have no physical links to this php include on the site.
Welcome to the toolbar. I have seen this often. Not so much include files, but files and domains that were never publicised.
If it is true PHP code that outputs html, there is really no way for google or anyone else to access it. Might need more info to understand what you mean.
It's an include which outputs html code - I have never linked to the actual php include file as a hyperlink...