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 the name of the class. We assign the...