If there is never an actual link to the included PHP script then strictly speaking a spider would have no knowledge of the filename and therefore never index it individually.
The content generated by the include will, of course, be indexed as part of the page the included it - there's nothing you can do about that unless you restrict crawling of the parent page via robots.txt.
If you really want to be sure; what I would do would be to put your include files into a separate directory, and then prohibit any access to that directory using .htaccess, for example, if you create your file in a subdirectory as "/includes/file.php", you could then create the following as "/includes/.htaccess":
Code:
deny from all
That would pretty much guarantee that no crawler will ever index includes/file.php on its own.
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