[Exim] Overwriting the Return-Path (PHP)

This might not be the right place to ask this but it's kinda a lost
resort and I seem to get answers here quickly.
My problem is overwriting the return path header with PHP. If I simply
specify "Return-Path: me@???" that won't do it. With
sendmail, I just have to use -f option such as mail("you@???",
"subject", "body", "Headers", "-fme@???") (notice the -f in the
fifth parameter). Does anyone know how to overwrite the Return-Path
headers with PHP on a server using Exim?

Thanks
- Richard S.