Autoload Classes in Core PHP

Hello all,

I am trying to understand the concept of autoloading using core PHP, not any framework.

I have a file named Donor.php which is under App -> src -> Contacts.

Here is the code of Donor.php

<?php namespace src\Contacts;

class Donor{ 

    public function __construct()
    {
        return ('Loaded');
    }

}

I have another file named namespace.php, which is immediate child of the root directory. The code of namespace.php looks like this.

<?php 

use src\Contacts;

new \app\Contacts\Donor;

My composer.json file looks like this.

{
    "autoload": {
        "psr-4":{
            "app\\" : "app/src"
        }
    }
}

Here is my full folder structure.

basic
   app
     src
          Billing
          Contacts
                Donor.php
   vendor
  composer.json
  namespace.php

Now wehenver i run namespace.php it always gives Fatal error: Class 'app\Contacts\Donor' not found in C:\xampp\htdocs\basic\namespace.php on line 5

Can anyone explain me what i am doing wrong here?