On the PHP Thinktank Blog, there's a new post that deals with dependency injection in PHP5 - a sort of tutorial on the topic, demonstrating with a simple example.
Several months ago I started using Spring Framework. That is how I faced the dependency injection. As my programming skills mostly originated from web development with PHP, I found out that I've missed a great technique, which is as usually common in Java land, but rare in PHP. I ended up with a research of dependency injection libraries for PHP.
However, all these solutions made you to define your dependencies within the code of your application, and were quite minimalistic. That made my mind - I started working on Garden - the dependency injection container for PHP5, which would use XML definitions with syntax as similar as possible to the Spring.
The example provided performs an injection to "put a collar on a dog", including the complete implementation, down to the output of the var_dump. He hints at some much more complex functionality than just this, though - potential for growth of the Garden library.