Change the email Subject?

I need to change the Emails Subject now. I see in mail.php where to change it, but I need to get a variable that I made, that exists on Checkout_confirm to be usable in mail.php.

I have made my own variable named $PO that I got to the email through the template, but the template(tpl) does not reach the emails subject.

Please help

Jason

Ok I found the email variable in Checkout_Confirm.php

Changed it and some text!