It shouldn't make any difference Thyrosis. The session cookie should be domain specific. The browser should recognise the difference between abc.com and zzz.com and only send the cookies related to each domain.
To be honest, I don't think there is anything wrong with the code that I can see. I think there might be something else going on with the browser or the configuration.
My helpful sig has gone because a mod below the administrator gave me an infraction - despite the administrator personally agreeing to it.
If you need any php tips or tricks you can PM me.