This is typically inserted by a control-revisioning system. At least, if you start with something like:

$version = '$Id$';
[download]

then when it's checked out, the ID will be expanded into the string above. Furthermore, the string will be updated on further checkouts.

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain