Learn

In real PHP projects you don't have one giant file — you have a namespaced class per file, autoloaded on demand. This is what makes Composer (and the entire modern PHP ecosystem) work.

Declaring a namespace

At the top of a file (must be the first non-comment statement):

<?php
namespace App\Services;

class UserService {
    public function find(int $id) { /* ... */ }
}

The class's fully-qualified name (FQN) is now \App\Services\UserService. Without a namespace declaration, classes go in the global namespace.

Using namespaced classes

Four ways to reference a class:

<?php
namespace App\Controllers;

use App\Services\UserService;            // import → use short name
use App\Services\PostService as Posts;   // alias

class HomeController {
    public function index() {
        $u = new UserService();              // imported short name
        $p = new Posts();                    // alias
        $x = new \App\Other\Thing();         // FQN with leading backslash
        $y = new \DateTimeImmutable();       // built-ins are global
    }
}

Always use a leading \ for global classes like \Exception or \DateTime when you're inside a namespace — without it, PHP looks in your namespace first.

PSR-4 autoloading

The PSR-4 standard maps namespace prefixes to directory paths. With composer.json:

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

The class App\Services\UserService lives at src/Services/UserService.php. Composer generates an autoloader that maps these — composer dump-autoload.

One class per file, filename matches class name (case-sensitive on Linux), directories match namespace segments. That's PSR-4.

use for functions and constants

Classes are imported by default. To import functions or constants explicitly:

use function App\Helpers\format;
use const App\MAX_SIZE;

Without use function, format(...) would look for it in the current namespace first, falling back to global.

Group use (PHP 7+)

use App\Services\{UserService, PostService, CommentService};

Cleaner than three separate use lines.

Common mistakes

• Forgetting the leading \ when calling built-ins inside a namespace — new DateTime() looks for App\Services\DateTime first.
• Mismatched filename/classname casing — works on macOS/Windows, breaks deploy on Linux.
• Putting namespace after other code — must be the first statement in the file.
• Trying to declare two namespaces in one file — legal but a code smell. Split them.