I'm trying NOT to do what you want.
I'm developing a site here on my localhost using sesison, and I open a new window (Ctrl+N) and go to our live system, I don't have to log in.
This means that I you only knew the name of the login session variable we use for our live system you could create the same on your computer at home and be automatically logged in to our live system :-)
So I do I define the domain/path or a session variable in PHP4??