PHP design pattern not working [on hold]

While reading a famous book on Design patterns. I tried to run this code. But it seems to be illogical, we are injecting Interface via the constructor? Thoughts.

class CustomerController {
 protected $repository;
 public function __construct(CustomerRepositoryInterface $repo) {
 $this->repository = $repo;
}
public function viewAction() {
 $customer = $this->repository->getById(1001);
 return $customer;
}