I often find files that are depreciated but never deleted, "just in case" something somewhere is still using them. This normally leads to files getting left for ages, and there is a risk that at some point someone will take over the project and have no idea they are depreciated.
In order to calm those fears I came up with a little PHP function to test if a script file is still being used.
< ?php
// This file has been depreciated
// TODO - Please remove for next verison unless the following email is recieved.
mail('you@example.com', 'Script Still In Use!', "The script " . __FILE__ . " is still being used!");
?>
It's handy to use the __file__ constant in PHP rather than any of the server variables so you get the script the code is in rather than the script the user has requested (if that makes sense).
This is by no means difinitive, it would be cool to have a stack to see what's calling the file, and some protection from getting 1000's of messages... in case it's included on a LOT of pages. However it is a nice little sanity check before you delete anything.