Challenge 3 — Solution

Task: Design (in comments/text, not necessarily runnable) a small
project structure with namespace App\Models containing a User class and
App\Services containing an EmailService class, following PSR-4 folder
conventions. Write the index.php that would use both via Composer's
autoloader, with no manual require_once for either class.

---- Project structure ----

my-app/
├── app/
│   ├── Models/
│   │   └── User.php          — namespace App\Models; class User
│   └── Services/
│       └── EmailService.php  — namespace App\Services; class EmailService
├── composer.json             — maps "App\\" to "app/"
├── vendor/                   — generated by Composer, not committed
└── index.php

---- app/Models/User.php ----

<?php
namespace App\Models;

class User {
    public function __construct(
        public string $name,
        public string $email
    ) {}
}

---- app/Services/EmailService.php ----

<?php
namespace App\Services;

use App\Models\User;

class EmailService {
    public function sendWelcomeEmail(User $user) {
        echo "Sending welcome email to {$user->email}...<br>";
    }
}

---- index.php ----

<?php
require __DIR__ . '/vendor/autoload.php';

use App\Models\User;
use App\Services\EmailService;

$user = new User("Alex", "alex@example.com");
$emailService = new EmailService();
$emailService->sendWelcomeEmail($user);
?>


Expected output:
Sending welcome email to alex@example.com...

Notes:
- Only ONE require appears anywhere in this project - vendor/
  autoload.php at the top of index.php. Neither User.php nor
  EmailService.php is ever require_once'd manually, because Composer's
  generated autoloader locates both automatically the moment
  "new User(...)" or "new EmailService()" is first referenced.
- EmailService.php itself uses "use App\Models\User;" to reference the
  User class inside its own method's type hint (sendWelcomeEmail(User
  $user)) - the same autoloading mechanism resolves this reference too,
  regardless of which file first triggers it.
- The folder structure directly mirrors the namespace structure per
  PSR-4: App\Models\User lives at app/Models/User.php, and
  App\Services\EmailService lives at app/Services/EmailService.php -
  exactly the predictable mapping that lets Composer's autoloader work
  without being told about each class individually.