# Dependency Injection

## Purpose

To implement a loosely coupled architecture in order to get better testable, maintainable and extendable code.

## Usage

Configuration gets injected and `Connection` will get all that it needs from `$config`. Without DI, the configuration would be created directly in `Connection`, which is not very good for testing and extending `Connection`.

Notice we are following Inversion of control principle in `Connection` by asking `$config` to implement `Parameters` interface. This decouples our components. We don't care where the source of information comes from, we only care that `$config` has certain methods to retrieve that information. Read more about Inversion of control [here](http://en.wikipedia.org/wiki/Inversion_of_control).

## Examples

* The Doctrine2 ORM uses dependency injection e.g. for configuration that is injected into a `Connection` object. For testing purposes, one can easily create a mock object of the configuration and inject that into the `Connection` object
* Symfony and Zend Framework 2 already have containers for DI that create objects via a configuration array and inject them where needed (i.e. in Controllers)