The objects themselves have a number of injected dependencies. It includes using the PHP magic methods __sleep and __wakeup to manage serialisation. It also discusses mechanisms for re-injecting dependencies on wakeup via a method that maintains Inversion of Control (IoC).
He uses a user object based example that has an injection needed to load in the user's usage history. He includes the code to define the classes, create the objects via a dependency injection container and performing the sleep/wakeup actions with the dependencies coming out intact on the other side.