An interface is an OOP element that groups a set of function declarations without implementing them, that is, it specifies the name, return type, and arguments, but not the block of code. Interfaces are different from abstract classes, since they cannot contain any implementation at all, whereas abstract classes could mix both method definitions and implemented ones. The purpose of interfaces is to state what a class can do, but not how it is done.

From our code, we can identify a potential usage of interfaces. Customers have an expected behavior, but its implementation changes depending on the type of customer. So, Customer could be an interface instead of an abstract class. But as an interface cannot implement any function, nor can it contain properties, we will have to move the concrete code from the Customer class to somewhere else. For now, let's move it up to the Person class. Edit the Person class as shown:

<?php

namespace Bookstore\Domain;

class Person {

    private...