You know that you cannot have two classes with the same name, since PHP would not know which one is being referred to when creating a new object. To solve this issue, PHP allows the use of namespaces, which act as paths in a filesystem. In this way, you can have as many classes with the same name as you need, as long as they are all defined in different namespaces. It is worth noting that, even though namespaces and the file path will usually be the same, this is enforced by the developer rather than by the language; you could actually use any namespace that has nothing to do with the filesystem.

Specifying a namespace has to be the first thing that you do in a file. In order to do that, use the namespace keyword followed by the namespace. Each section of the namespace is separated by , as if it was a different directory. If you do not specify the namespace, the class will belong to the base namespace, or root. At the beginning of both files—Book.php and Customer.php—add the following:

<?php

namespace BookstoreDomain;

The preceding line of code sets the namespace of our classes as BookstoreDomain. The full name of our classes then is BookstoreDomainBook and BookstoreDomainCustomer. If you try to access the init.php file from your browser, you will see an error saying that either the class Book or the class Customer were not found. But we included the files, right? That happens because PHP thinks that you are trying to access Book and Customer from the root. Do not worry, there are several ways to amend this.

One way would be to specify the full name of the classes when referencing them, that is, using $customer = new BookstoreDomainBook(); instead of $book = new Book();. But that does not sound practical, does it?

Another way would be to say that the init.php file belongs to the BookStoreDomain namespace. That means that all the references to classes inside init.php will have the BookStoreDomain prefixed to them, and you will be able to use Book and Customer. The downside of this solution is that you cannot easily reference other classes from other namespaces, as any reference to a class will be prefixed with that namespace.

The best solution is to use the keyword use. This keyword allows you to specify a full class name at the beginning of the file, and then use the simple name of the class in the rest of that file. Let's see an example:

<?php

use BookstoreDomainBook;
use BookstoreDomainCustomer;

require_once __DIR__ . '/Book.php';
require_once __DIR__ . '/Customer.php';
//...

In the preceding file, each time that we reference Book or Customer, PHP will know that we actually want to use the full class name, that is, with BookstoreDomain prefixed to it. This solution allows you to have a clean code when referencing those classes, and at the same time, to be able to reference classes from other namespaces if needed.

But what if you want to include two different classes with the same name in the same file? If you set two use statements, PHP will not know which one to choose, so we still have the same problem as before! To fix that, either you use the full class name—with namespace—each time you want to reference any of the classes, or you use aliases.

Imagine that we have two Book classes, the first one in the namespace BookstoreDomain and the second one in LibraryDomain. To solve the conflict, you could do as follows:

use BookstoreDomainBook;
use LibraryDomainBook as LibraryBook;

The keyword as sets an alias to that class. In that file, whenever you reference the class LibraryBook, you will actually be referencing the class LibraryDomainBook. And when referencing Book, PHP will just use the one from Bookstore. Problem solved!