multiple domains using one cookie

Hello all. I want multiple domains to be able to read from one cookie, or set one cookie each for the different domains. Here is the case: When the user logs in to my server, I want to set a cookie that has the value $name="tobias". He specifies which domains that can read this cookie, something like $domains="domain1.com;domain2.com;domain6.com". Then these sites can read the $name and print it. The other way might be to set one cookie with the value $name="tobias" for domain1.com, one for domain2.com and one for domain6.com. But the cookie _has_ to come from domain1.com, otherwise it will only work for mydomain.com, right? In other words, I can´t set, edit or read a cookie for an other site? How would I go about doing this? Regards, // Tobias