On the TutsPlus.com site today they've posted a tutorial sharing some examples of dependency injection using the Symfony
In this article, we'll look at some examples of using the Symfony DependencyInjection component. You'll learn the basics of dependency injection, which allows cleaner and more modular code, and you'll see how to use it in your PHP application with the Symfony component.
The tutorial starts by introducing the component and what kind of functionality it includes to help register and fetch services on demand. It then walks through the installation (via Composer) and the installation of other optional related packages. It then dives into the code, showing how to put the package to use to register a new service and pull it back out to use. The article then moves on and shows a more "real world" example with services that have dependencies and configuring it in a YAML configuration file.