Fabien Potencier has posted the most recent article in his "Symfony Service Container" series, a look at using XML/YAML to describe services.
Today, with the help of service loaders and dumpers, you will learn how to use XML or YAML to describe your services. The Symfony Dependency Injection component provides helper classes that load services using "loader objects". By default, the component comes with two of them: sfServiceContainerLoaderFileXml to load XML files, and sfServiceContainerLoaderFileYaml to load YAML files.
He reviews the "dumper objects" - tools used to take a service container and push it out into normal PHP code - and how you can use them to dump the Service Container's information out to the XML and YAML formats. Once you have this, it can be loaded back at any time via the two loaders mentioned above. There's plenty of code examples included for these and other more detailed examples.