Lesson 6 - Dependency Injection and Test doubles

In the previous lesson, PHP Testing - Xdebug and coverage reports, we learned to work with Xdebug and generate code-coverage reports. In today's PHP tutorial, we're going to start with the last topic associated with unit tests and that's mocking.

Motivation

Our unit tests worked only with simple classes that had no dependencies so far. But how to test classes that need some other class instances to work and even these instances may need another instances of other classes? We often talk about the System Under Test (SUT), which refers to just a small part of a larger system we're currently testing. As for our calculator, we can imagine it needs some logger to work, which stores computed problems into a file. That still wouldn't be such a problem, we could test the logger as well, and the calculator test would normally log the problems and we could just live with that. But how would we test classes that work with a web API? For example, the calculator would share the results on Facebook. We certainly don't want to run such a "madness" in tests and share something on our Facebook at the end of the tests. Tests shouldn't even require an Internet connection unless it's our intention to test the communication between our own APIs with the integration tests. Here we use mocking that allows us to create "fake objects", for example, an object that behaves as an instance of the FacebookApi class, but it actually provides prepared "fake" data only and calling its methods won't cause a real requests to Facebook.

Dependency injection

To be able to use mocking, which means to provide fake objects in our tests, we need to use Dependency injection. It's a design pattern that is so important that we can

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