Build a Professional Clean Code SaaS Application

/src
  /Domain             (Inner Ring: Entities and Interfaces. ZERO dependencies)
    /Entities         (e.g., Subscription, User)
    /Repositories     (Interfaces only, e.g., SubscriptionRepositoryInterface)
  /UseCases           (Middle Ring: Application Business Rules)
    ProcessSubscriptionUseCase.php
  /Infrastructure     (Outer Ring: Databases, APIs, Framework specific)
    /Database
      MySQLSubscriptionRepository.php
    /Http
      /Controllers
        SubscriptionController.php

namespace App\Domain\Entities;

class Subscription {
    private string $id;
    private string $status;
    private float $monthlyRate;

    public function __construct(string $id, float $monthlyRate) {
        $this->id = $id;
        $this->status = 'ACTIVE';
        $this->monthlyRate = $monthlyRate;
    }

    // Encapsulated Behavior (No public setters!)
    public function cancel(): void {
        $this->status = 'CANCELED';
    }

    public function isChargeable(): bool {
        return $this->status === 'ACTIVE' && $this->monthlyRate > 0;
    }
}

namespace App\Domain\Repositories;
use App\Domain\Entities\Subscription;

interface SubscriptionRepositoryInterface {
    public function findById(string $id): ?Subscription;
    public function save(Subscription $subscription): void;
}

namespace App\UseCases;
use App\Domain\Repositories\SubscriptionRepositoryInterface;
use App\Domain\PaymentGatewayInterface;

class ProcessSubscriptionUseCase {
    private $repository;
    private $paymentGateway;

    // DIP: We inject interfaces, not concrete classes.
    public function __construct(
        SubscriptionRepositoryInterface $repository,
        PaymentGatewayInterface $paymentGateway
    ) {
        $this->repository = $repository;
        $this->paymentGateway = $paymentGateway;
    }

    public function execute(string $subscriptionId): void {
        $subscription = $this->repository->findById($subscriptionId);
        
        // Guard Clause (Fail Fast)
        if (!$subscription) {
            throw new Exception("Subscription not found.");
        }

        // Clean, readable business logic
        if ($subscription->isChargeable()) {
            $this->paymentGateway->charge($subscription->getMonthlyRate());
        }
    }
}

namespace App\Infrastructure\Http\Controllers;
use App\UseCases\ProcessSubscriptionUseCase;

class SubscriptionController {
    private $processUseCase;

    public function __construct(ProcessSubscriptionUseCase $useCase) {
        $this->processUseCase = $useCase;
    }

    public function charge(string $id) {
        try {
            $this->processUseCase->execute($id);
            return json_encode(["status" => "success"]);
        } catch (Exception $e) {
            return json_encode(["status" => "error", "message" => $e->getMessage()]);
        }
    }
}