Igor Wiedler has a recent post to his site about creating stateless services, specifically in the context of using a dependency injection container to manage the objects your application uses.

As more frameworks and libraries, particularly in the PHP world, move towards adopting the Dependency Injection pattern they are all faced with the problem of bootstrapping their application and constructing the object graph. In many cases this is solved by a Dependency Injection Container (DIC). Such a container manages the creation of all the things. The things it manages are services. Or are they?

He notes that, according to some of the principles of domain-driven design, "services" should be stateless - the results of calls to the service shouldn't alter it, it should only depend on the values passed in. He goes on to put this into the context of a DIC and gives an example of the "request service" (and how it violates the DDD principles of statelessness). He talks some about scopes (dependencies) and mutable services. He talks about methods to get around these issues with the "request" instance, ultimately coming to the conclusion that event listeners might be the way to go.

On the PHPFreaks website, there's a new tutorial talking about sessions and cookies in PHP:

HTTP is a stateless protocol. This means that each request is handled independently of all the other requests and it means that a server or a script cannot remember if a user has been there before. However, knowing if a user has been there before is often required and therefore something known as cookies and sessions have been implemented in order to cope with that problem.

The tutorial is pretty introductory, so if you're not new to the PHP world, you won't learn much. New developers, though, will learn how to set cookies, use sessions and learn a bit about the security of both.