阻止$ _SESSION变量在同一台服务器上的多个域上？

I am in the process of planning for a series of websites that will all point to the same server and share the same back end code, but have different domain names, different content, and look completely different. Here is my issue, say a user visits one website and adds something to the cart, then visits the other website. (which is actually the same server just different domain pointing to it) I do not want the SESSION variables to travel across. I am in the process of planning for a series of