Separation of Concerns (SoC)

// Bad - Mixed concerns
class User {
    public function save() {
        $db = new PDO('mysql:host=localhost;dbname=app', 'user', 'pass');
        $stmt = $db->prepare("INSERT INTO users (name, email) VALUES (?, ?)");
        $stmt->execute([$this->name, $this->email]);
    }
}

// Good - Separated database logic
class User {
    private string $name;
    private string $email;
}

class UserRepository {
    private PDO $db;

    public function save(User $user) {
        $stmt = $this->db->prepare("INSERT INTO users (name, email) VALUES (?, ?)");
        $stmt->execute([$user->getName(), $user->getEmail()]);
    }
}

// Bad - Mixed validation and business logic
class Order {
    public function process() {
        if (empty($this->items)) {
            throw new Exception('Order cannot be empty');
        }
        if ($this->total < 0) {
            throw new Exception('Invalid total amount');
        }
        // Process order...
    }
}

// Good - Separated validation
class OrderValidator {
    public function validate(Order $order): array {
        $errors = [];
        if (empty($order->getItems())) {
            $errors[] = 'Order cannot be empty';
        }
        if ($order->getTotal() < 0) {
            $errors[] = 'Invalid total amount';
        }
        return $errors;
    }
}

class Order {
    public function process() {
        // Only handles order processing
    }
}

// Bad - Mixed HTML and logic
class ProductPage {
    public function show($id) {
        $product = $this->getProduct($id);
        echo "<h1>{$product->name}</h1>";
        echo "<p>Price: ${$product->price}</p>";
    }
}

// Good - Separated presentation
class ProductController {
    public function show($id) {
        $product = $this->productRepository->find($id);
        return $this->view->render('product/show', ['product' => $product]);
    }
}

// product/show.php template
<h1><?= htmlspecialchars($product->name) ?></h1>
<p>Price: $<?= htmlspecialchars($product->price) ?></p>

// Bad - Mixed business logic
class OrderController {
    public function checkout() {
        $order = new Order($_POST['items']);
        $payment = new Payment($_POST['card']);
        $payment->process();
        $order->updateStatus('paid');
        $email = new EmailService();
        $email->sendConfirmation($order);
    }
}

// Good - Separated services
class OrderService {
    private PaymentService $paymentService;
    private EmailService $emailService;

    public function processOrder(Order $order, PaymentData $paymentData): void {
        $this->paymentService->process($paymentData);
        $order->updateStatus('paid');
        $this->emailService->sendConfirmation($order);
    }
}

class OrderController {
    public function checkout() {
        $this->orderService->processOrder($order, $paymentData);
    }
}

// Bad - Hardcoded configuration
class EmailSender {
    private $host = 'smtp.example.com';
    private $port = 587;

    public function send($message) {
        // Sending logic using hardcoded values
    }
}

// Good - Separated configuration
// config/mail.php
return [
    'host' => 'smtp.example.com',
    'port' => 587
];

class EmailSender {
    private array $config;

    public function __construct(array $config) {
        $this->config = $config;
    }

    public function send($message) {
        // Sending logic using config values
    }
}