Namespaces & PSR-4

Namespaces are folders for class names. PSR-4 is the agreement that maps those names to real files. Composer makes it automatic - like a nicer import map.

Why namespaces

Without them, every class is global. Two packages both shipping Router = pain.

<?php

declare(strict_types=1);

namespace App\Http;

class Request
{
    // full name: App\Http\Request
}

use statements

<?php

declare(strict_types=1);

namespace App\Controllers;

use App\Http\Request;
use App\Http\Response;
use App\Services\UserService;
use function App\Support\str_slug;
use const App\Config\APP_NAME;

class UserController
{
    public function __construct(private UserService $users) {}

    public function show(Request $request): Response
    {
        // ...
    }
}

Alias collisions:

<?php

use App\Models\User as UserModel;
use Vendor\Auth\User as AuthUser;

PSR-4 mapping (the important bit)

In composer.json:

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

Meaning:

Class	File
App\Http\Request	src/Http/Request.php
App\Services\UserService	src/Services/UserService.php

Rules of thumb:

• Namespace segment ↔ directory segment
• Class name ↔ file name (case-sensitive on Linux!)
• One public class per file (convention; do it)

After changing autoload config:

composer dump-autoload

How a request finds your class

Every entry script starts with:

<?php

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

Then you use / new freely. No manual requires for your app classes.

Global namespace & leading slash

<?php

namespace App;

$dt = new \DateTimeImmutable(); // leading \ = from global namespace

Inside a namespace, unqualified DateTimeImmutable looks for App\DateTimeImmutable. The leading \ or a use DateTimeImmutable; fixes it.

Folder shape that transfers to Laravel/Symfony

src/
  Http/
    Controllers/
    Middleware/
  Models/          # or Domain/, Entity/ in Symfony
  Services/
  Support/
public/
  index.php        # front controller - requires autoload

Frameworks rearrange names (app/ in Laravel, src/ in Symfony) but the PSR-4 idea is identical.

Once namespaces click, Composer is just "npm for this mapping + packages".

Best,

-- ł.