Parent File Constant?

I've been looking through the list archives and the manual, but can't find out... how to get the name of the parent file (from an include statement). Example: main.php includes "header.php" and "footer.php". How does header.php (or footer.php) know that it was called by main.php, and not some other file? Is there a php constant or function that returns the parent file path/name? Or do I have to set a variable before my include statement? Thanks, mario