如何为匿名用户提供ID以记住他们在数据库中的数据？

I want to give an ID to a user that opens my website and adds some items to their "shopping cart", even if they aren't signed up to the service. Just add, then proceed to sign up when they want to check out. And if they don't go to checkout after adding stuff, then close their browser, come back 2 days later, I want to retrieve their previous orders from database.I want to give an ID to a user that opens my we