mail return-path problem

I was just curious if there was a way to set the return path of an email dynamically. I've looked around and all I could find was a Zend tutorial running sendmail from the command line, which I don't want to do. :) I tried setting "Return-Path:" in the mail() headers, but that didn't seem to make a difference. If anybody knows anything about this and could point me in the right direction, I'd appreciate it. Thanks!