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# Dependency Injection (PHP-DI)

Automated testing is great, but whether it’s easy to write or not depends on your code design. As a rule of thumb, the more loosely coupled your classes are, the easier they are to test. And one can’t talk about loosely coupled classes and not mention [dependency injection](https://en.wikipedia.org/wiki/Dependency_injection).

One simple example should clarify how it works. Imagine a class that encountered an error and wants to log it. Let’s also assume you’re using [Monolog ](https://packagist.org/packages/monolog/monolog)to create loggers that handle these events. Without dependency injection, whenever the class encounters an error it must now instantiate a logger and use it. If you have 50 classes that use loggers, you would write the code for instantiating a logger 50 times in 50 places (well, you’d probably write a function somewhere to simply retrieve an instantiated logger and that too would be a sort of dependency injection, in a nutshell).

With dependency injection, you would create a logger at some point in time and then simply pass it along to each class that can log errors (traditionally, in the constructor or through a method like `set_logger`). That way your classes don’t actually know what a logger is or how it works or how to get one — they simply get one passed along.

Normally you wouldn’t even bind yourself to any specific logger but you would use an interface. Luckily, for loggers, there already exists the [PSR-3](https://www.php-fig.org/psr/psr-3/) standard (which Monolog also implements). So your classes simply expect a PSR-3 `LoggerInterface` object passed on in the constructor.

When you write automatic tests for your classes, you don’t need to actually load Monolog too! You can just pass on any PSR-3 implementing class, or just use [Mockery ](https://github.com/mockery/mockery)to create one on the fly.

Dependency injection containers usually provide an array of features that are much more powerful than just that (e.g., auto-wiring or on-the-fly instantiation). For our projects, we use [PHP-DI](https://php-di.org/) but this is not actually a requirement for using our framework. The framework accepts any [PSR-11](https://www.php-fig.org/psr/psr-11/)-compatible container and only assumes the features that PSR-11 foresees.