On the SitePoint PHP blog there's a tutorial posted giving you a "fresh look at dependency injection" using the Disco library from bitExpert.

Dependency Injection is all about code reusability. It’s a design pattern aiming to make high-level code reusable, by separating the object creation / configuration from usage.

[...] Our very own Alejandro Gervasio has explained the DI concept fantastically, and Fabien Potencier also covered it in a series. There’s one drawback to this pattern, though: when the number of dependencies grows, many objects need to be created/configured before being passed into the dependent objects. [...] In this article, we’ll demonstrate the concept [of a dependency injection container] further with a newcomer in this field: Disco.

He starts with the installation of the library via Composer and starting up the built-in PHP web server for testing. With that up and running he shows you how to create an instance of the container and set up a sample service with the configuration in the docblock comments. The tutorial then talks about the service scopes, container parameters and shows it in action to create a simple controller handling frontend. There's a bit more about configuration, the container builder functionality and the creation of a response listener when routes are requested.