1// Bad - Multiple responsibilities
2class User {
3    public function save() { /* database logic */ }
4    public function sendEmail() { /* email logic */ }
5    public function generateReport() { /* reporting logic */ }
6}
7
8// Good - Single responsibility
9class User {
10    public function __construct(
11        public string $name,
12        public string $email
13    ) {}
14}
15
16class UserRepository {
17    public function save(User $user): void {
18        // Database logic only
19    }
20}

1class OrderProcessor {
2    public function __construct(
3        private readonly PaymentGateway $gateway,
4        private readonly NotificationService $notifier,
5        private readonly Logger $logger
6    ) {}
7    
8    public function process(Order $order): void {
9        try {
10            $this->gateway->charge($order->total);
11            $this->notifier->sendConfirmation($order);
12            $this->logger->info('Order processed');
13        } catch (PaymentException $e) {
14            $this->logger->error('Payment failed');
15            throw $e;
16        }
17    }
18}

1interface UserRepositoryInterface {
2    public function find(int $id): ?User;
3    public function findByEmail(string $email): ?User;
4    public function save(User $user): void;
5}
6
7class DatabaseUserRepository implements UserRepositoryInterface {
8    public function __construct(
9        private readonly PDO $pdo
10    ) {}
11    
12    public function find(int $id): ?User {
13        $stmt = $this->pdo->prepare('SELECT * FROM users WHERE id = ?');
14        $stmt->execute([$id]);
15        $data = $stmt->fetch();
16        return $data ? User::fromArray($data) : null;
17    }
18}

1interface PaymentGatewayFactory {
2    public function create(string $type): PaymentGateway;
3}
4
5class PaymentGatewayFactoryImpl implements PaymentGatewayFactory {
6    public function create(string $type): PaymentGateway {
7        return match($type) {
8            'stripe' => new StripeGateway(),
9            'paypal' => new PayPalGateway(),
10            'square' => new SquareGateway(),
11            default => throw new InvalidArgumentException("Unknown gateway")
12        };
13    }
14}