Ralph Schindler has a new post to his blog describing some of the what he's done to understand dependency injection in PHP and includes some illustrations to share that information.
Whatever the reason, one of the newest concepts (at least over the past 3 years or so) that has emerged as one of our heated topics of debate is how to manage object dependencies. Interestingly, the argument of how to manage dependencies is generally named by the solution which it's proponents give as the solution: dependency injection (the abstract principle is actually called Inversion of control).
He starts with simple examples - pushing an object into the creation of another - and backfills with a brief history of dependencies in PHP applications for a little context. He talks about DI frameworks like Spring and some of the other criticisms and thoughts you'll need to consider when using DI (and DI containers) during development and deployment.