# Namespaces, Autoloading and Composer Packages — PHP

Source: https://www.skillbyai.com/en/php/e-namespaces

> Organise code with namespaces and PSR-4 autoloading, and manage dependencies.

## From include files to packages

Old PHP code used long chains of `require` statements. Modern PHP organises classes into **namespaces** that mirror directories: `namespace Shop\Orders;` in `src/Orders/`. Import names with **`use`** statements (`use Shop\Payments\PaymentGateway;`), with aliases when names clash. **PSR-4 autoloading** maps a namespace prefix to a directory, so `Shop\Orders\OrderService` loads `src/Orders/OrderService.php` automatically the first time it is used; Composer generates the autoloader, and you include `vendor/autoload.php` once at your entry point. Keep **one class per file**, with the file name matching the class name. Third-party code comes from **Packagist** via Composer, using **semantic version constraints**: `^3.2` allows 3.x updates but not 4.0. Run `composer audit` to check installed packages for known vulnerabilities, `composer outdated` to see available updates, and keep `require-dev` packages (PHPUnit, PHPStan) out of production installs with `composer install --no-dev`.

## Namespaces with PSR-4 autoloading

The directory layout matches namespaces, so no require statements are needed.

```php
// composer.json
// {
//   "require": { "php": "^8.3", "monolog/monolog": "^3.5" },
//   "autoload": { "psr-4": { "Shop\\": "src/" } }
// }

// src/Orders/OrderService.php
<?php
declare(strict_types=1);

namespace Shop\Orders;

use Psr\Log\LoggerInterface;
use Shop\Payments\PaymentGateway;

final class OrderService
{
    public function __construct(
        private readonly PaymentGateway $payments,
        private readonly LoggerInterface $logger,
    ) {}
}

// public/index.php
<?php
require __DIR__ . '/../vendor/autoload.php';
$service = new Shop\Orders\OrderService($gateway, $logger);   // loaded automatically

// composer audit        # known vulnerabilities in dependencies
```

## Type against PSR interfaces

Depend on `Psr\Log\LoggerInterface`, `Psr\Http\Client\ClientInterface` or `Psr\SimpleCache\CacheInterface` instead of a specific library class. You can then swap Monolog for another logger without touching your code.

**Quiz:** With PSR-4 mapping "Shop\\" to "src/", where should the class Shop\Orders\OrderService live?

- [x] src/Orders/OrderService.php
- [ ] src/OrderService.php
- [ ] Shop/Orders.php
- [ ] vendor/Shop/OrderService.php

*Answer:* src/Orders/OrderService.php. PSR-4 maps the namespace segments after the prefix onto directories under src/.