Allan MacGregor has a recent post to his site looking at the use of Dependency Injection in PHP applications. He covers two main methods, constructor injection and setter injection, and code examples of each.
By using Dependency Injection we can write more maintainable, testable, and modular code. All projects have dependencies. The larger the project the more dependencies is it bound to have; now having a great number of dependencies is nothing bad by itself however how those dependencies are managed and maintained is.
Dependency Injection is not a new pattern and it has been commonly used on many languages like Java, but this pattern is somewhat new in the PHP world and it's gaining traction quickly thanks for frameworks like laravel.
He starts with a class that has no dependency injection, hard-coding dependencies inside the constructor. His first refactor shows how to take this class and use constructor injection to pass in an instance of an object rather than a value. After briefly explaining this method, he moves on to setter injection. He shows how to use a "setter" method to inject the needed object.