Classes and objects

Objects are representations of real-life elements. Each object has a set of attributes that differentiates it from the rest of the objects of the same class, and is capable of a set of actions. A class is the definition of what an object looks like and what it can do, like a pattern for objects.

Let's take our bookstore example, and think of the kind of real-life objects it contains. We store books, and let people take them if they are available. We could think of two types of objects: books and customers. We can define these two classes as follows:

<?php

class Book {
}

class Customer {
}

A class is defined by the keyword class followed by a valid class name—that follows the same rules as any other PHP label, like variable names—and a block of code. But if we want to have a specific book, that is, an object Book—or instance of the class Book—we have to instantiate it. To instantiate an object, we use the keyword new followed by...