On his blog today Jakub Zalas has posted a tutorial he's written up about using dependency injection in PHP with the Symfony2 dependency injection component (DIC).
Symfony's DependencyInjection component is a PHP implementation of a Service Container, or as others like to call it, a Dependency Injection Container (DIC). The component also provides useful tools for handling service definitions, like XML loaders or dumpers. If you want to learn more about the dependency injection or the dependency injection container, read an excellent series of articles on the subject by Fabien Potencier: What is Dependency Injection?
He walks you through the entire process - installing the needed libraries (the DIC, a config and class loader component and Buzz, a lightweight HTTP client). Code is included to show object creation the "usual way" and then creating the same types of objects in a dependency injection environment. Also included is a sample XML document describing the services for the container. He finishes the post with a GraphvizDumper-generated image for the container.