I am working on multiple sites that I would like to share the same login system (much like Windows Live).

What's the best way to accomplish this via PHP? I thought about creating some sort of web service that you can authenticate against but I'm not sure if that's the best way to go. I've never created a web service in PHP before and it seems quite complex and I was hoping for a simple yet secure and fast way of doing this (Xml serialization and deserialization, IMO, isn't necessary if the site that's authenticating is on the same server so I would prefer to not use this).

Have any of you created a system like this?