How does the keyword "use" work in PHP and can I import classes with it?

Question

No, you can not import a class with the use keyword. You have to use include/require statement. Even if you use a PHP auto loader, still autoloader will have to use either include or require internally.

The Purpose of use keyword:

Consider a case where you have two classes with the same name; you'll find it strange, but when you are working with a big MVC structure, it happens. So if you have two classes with the same name, put them in different namespaces. Now consider when your auto loader is loading both classes (does by require), and you are about to use object of class. In this case, the compiler will get confused which class object to load among two. To help the compiler make a decision, you can use the use statement so that it can make a decision which one is going to be used on.

Nowadays major frameworks do use include or require via composer and psr

1) composer

2) PSR-4 autoloader

Going through them may help you further. You can also use an alias to address an exact class. Suppose you've got two classes with the same name, say Mailer with two different namespaces:

namespace SMTP;
class Mailer{}

and

namespace Mailgun;
class Mailer{}

And if you want to use both Mailer classes at the same time then you can use an alias.

use SMTP\Mailer as SMTPMailer;
use Mailgun\Mailer as MailgunMailer;

Later in your code if you want to access those class objects then you can do the following:

$smtp_mailer = new SMTPMailer;
$mailgun_mailer = new MailgunMailer;

It will reference the original class.

Some may get confused that then of there are not Similar class names then there is no use of use keyword. Well, you can use __autoload($class) function which will be called automatically when use statement gets executed with the class to be used as an argument and this can help you to load the class at run-time on the fly as and when needed.

Refer this answer to know more about class autoloader.

use doesn't include anything. It just imports the specified namespace (or class) to the current scope

If you want the classes to be autoloaded - read about autoloading

Don’t overthink what a Namespace is.

Namespace is basically just a Class prefix (like directory in Operating System) to ensure the Class path uniqueness.

Also just to make things clear, the use statement is not doing anything only aliasing your Namespaces so you can use shortcuts or include Classes with the same name but different Namespace in the same file.

E.g:

// You can do this at the top of your Class
use Symfony\Component\Debug\Debug;

if ($_SERVER['APP_DEBUG']) {
    // So you can utilize the Debug class it in an elegant way
    Debug::enable();
    // Instead of this ugly one
    // \Symfony\Component\Debug\Debug::enable();
}

If you want to know how PHP Namespaces and autoloading (the old way as well as the new way with Composer) works, you can read the blog post I just wrote on this topic: https://enterprise-level-php.com/2017/12/25/the-magic-behind-autoloading-php-files-using-composer.html

You'll have to include/require the class anyway, otherwise PHP won't know about the namespace.
You don't necessary have to do it in the same file though. You can do it in a bootstrap file for example. (or use an autoloader, but that's not the topic actually)

The issue is most likely you will need to use an auto loader that will take the name of the class (break by '\' in this case) and map it to a directory structure.

You can check out this article on the autoloading functionality of PHP. There are many implementations of this type of functionality in frameworks already.

I've actually implemented one before. Here's a link.