The first thing we need to understand is that if we create objects using new inside the controller, we will not be able to mock them. This means that we need to inject all the dependencies—for example, using a dependency injector. We will do this for all of the dependencies but one: the models. In this section, we will test the borrow method of the BookController class, so we will show the changes that this method needs. Of course, if you want to test the rest of the code, you should apply these same changes to the rest of the controllers.

The first thing to do is to add the BookModel instance to the dependency injector in our index.php file. As this class also has a dependency, PDO, use the same dependency injector to get an instance of it, as follows:

$di->set('BookModel', new BookModel($di->get('PDO')));

Now, in the borrow method of the BookController class, we will change the new instantiation of the model to the following:

public function borrow(int $bookId): string {
    $bookModel = $this->di->get('BookModel');

    try {
//...