I tried using Cookie: PHPSESSID=bf466c8240e4beabe8f18f46d8a49b56 , but that didn't work either.
You have to return the same ID (hash) as you get from the server. It does work. It's how your browser does it. You can look through your browser cache for the specific cookie. It'll be named something like PHPSESSID@.... Also, if using Firefox, install the LiveHTTPheaders plug in, and you can see exactly what's going on, and perhaps more importantly, what's not going on.