Hello!
root@a87970b52d5b:/app#
.
|-- index.php
`-- src
    `-- User
        `-- User.php



---------------------------------------------------
index.php
---------------------------------------------------

<?php

spl_autoload_register(function (string $class) {
    $prefix = 'App\\';
    $baseDir = __DIR__ . '/src/';

    if (strncmp($prefix, $class, strlen($prefix)) !== 0) {
        return;
    }

    $relativeClass = substr($class, strlen($prefix));

    $file = $baseDir . str_replace('\\', '/', $relativeClass) . '.php';

    if (file_exists($file)) {
        require $file;
    }
});

use App\User\User;

$user = new User();

echo $user->hello();

---------------------------------------------------
src/User/User.php
---------------------------------------------------

<?php

namespace App\User;

class User
{
    public function hello(): string
    {
        return 'Hello!';
    }
}