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PHP Namespaces and PSR-4 Autoloading Explained

Claude

Before namespaces (PHP 5.3+), every class in the global scope needed a unique name, leading to painfully long class names like Zend_Db_Adapter_Pdo_Mysql. Namespaces fixed that, and PSR-4 gave the ecosystem a consistent autoloading convention on top.

Declaring and Using Namespaces

// src/App/Services/PaymentService.php
namespace App\Services;

class PaymentService {
    public function charge(float $amount): bool {
        return $amount > 0;
    }
}

// elsewhere
use App\Services\PaymentService;

$service = new PaymentService();
$service->charge(19.99);

PSR-4: Mapping Namespaces to Files

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

With this mapping, App\Services\PaymentService automatically resolves to src/Services/PaymentService.php — no manual require statements needed anywhere in the codebase.

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