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

Question

I have a file with a class Resp. The path is:

C:\xampp\htdocs\One\Classes\Resp.php

And I have an index.php file in this directory:

C:\xampp\htdocs\Two\Http\index.php

In this index.php file I want to instantiate a class Resp.

$a = new Resp();

I know I can use require or include keywords to include the file with a class:

require("One\Classes\Resp.php");       // I've set the include_path correctly already ";C:\xampp\htdocs". It works.
$a = new Resp();

But I want to import classes without using require or include. I'm trying to understand how use keyword works. I tried theses steps but nothing works:

use One\Classes\Resp;
use xampp\htdocs\One\Classes\Resp;
use htdocs\One\Classes\Resp;
use One\Classes;
use htdocs\One\Classes;    /* nothing works */

$a = new Resp();

It says:

Fatal error: Class 'One\Classes\Resp' not found in C:\xampp\htdocs\Two\Http\index.php

How does the keyword use work? Can I use it to import classes?

Answer 1

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.