Namespaces and PSR-4 Autoloading — PHP 8: Modern PHP Development

Q: With PSR-4 mapping App\\ => src/, where is App\Http\Controllers\HomeController?

In `src/Http/Controllers/HomeController.php`. The prefix is stripped and remaining namespace segments become directories.

Q: Why might autoloading work locally but fail in a Linux container?

Linux file systems are case-sensitive. A file named `Homecontroller.php` matches `HomeController` on macOS/Windows but not on Linux.

Namespaces prevent naming collisions and group related code (App\Billing\Invoice). PSR-4 maps a namespace prefix to a directory: with "App\\": "src/", the class App\Billing\Invoice lives in src/Billing/Invoice.php. Composer generates an autoloader, so you only require 'vendor/autoload.php' once in your entry point. Use use statements to import classes, functions and constants.

<?php
// src/Billing/InvoiceService.php
declare(strict_types=1);

namespace App\Billing;

use App\Billing\Exception\InvoiceNotFound;
use Psr\Log\LoggerInterface;

final class InvoiceService
{
    public function __construct(private InvoiceRepository $invoices, private LoggerInterface $logger) {}
}

// public/index.php
require __DIR__ . '/../vendor/autoload.php';
$service = $container->get(App\Billing\InvoiceService::class);

Common pitfalls

File path casing not matching the class name — works on macOS, fails on Linux.
Forgetting composer dump-autoload after changing autoload config.
Putting multiple classes in one file under PSR-4.

Key points

One class per file, path mirrors namespace
Require Composer's autoloader once
Use optimized/authoritative class maps in production

Common pitfalls

Key points

Practice questions