PHP - How can I tell in which domain a script will look for cookies?

I'm trying to fix a really elusive bug where I lose my session upon redirect. In my local setup, there is no problem; it only manifests itself on the live site. I discovered that my local version was creating two identical cookies: one for .mydomain.com and one for www.mydomain.com. However, the live site only creates a cookie for .mydomain.com. It seems as though my script is looking for the 'www.mydomain.com' cookie, which is why there's no apparent problem on the local server.

How do I determine to which domain a given script will look for its cookies? (or if this sounds like a different problem, please let me know)