title: Secure and Flexible Cross-Domain Sessions
body: <p>I am having an issue that I hope you can help with. Let's say I work for a hypothetical company called "Blammo", and we have a hypothetical product called "Log". I am trying to set up a system where someone could log in to logfromblammo.com and order some of our products, and then when they are ready to purchase go to checkout.blammo.com to pay for their order. Eventually I want to allow for Blammo to launch a new hypothetical product with it's own website: rockfromblammo.com, and have that site also able to share a session with checkout.blammo.com so that users can have a single shopping cart across both product websites.</p>
  
  <p>Naturally the hypothetical scenario described above is not how my company actually works, but it is a fair example of what I need to do. We have an existing user database, and we have ways to authenticate any of our users on any of our sites, but the goal I have is to allow users to cross seamlessly from one site to another without having to re-authenticate. This would also allow for us to seamlessly transfer data such as a shopping cart to the checkout site.</p>
  
  <p>I have (briefly) looked at solutions such as OpenID, but I need to be able to integrate whatever solution we have with our existing authentication method, which is not terribly robust. Is there any good way to do this through PHP alone?</p>