DDD Knowledge Base

Quick reference for DDD architecture patterns and PHP implementation guidelines.

Core Principles

Layer Dependencies (Clean Architecture)

Presentation → Application → Domain ← Infrastructure
                    ↓
              Domain (center)

Rule: Dependencies point INWARD. Domain has ZERO external dependencies.

Layer Responsibilities

Layer	Contains	Depends On
Domain	Entities, Value Objects, Aggregates, Domain Services, Repository Interfaces, Domain Events	Nothing
Application	Use Cases, DTOs, Application Services	Domain
Infrastructure	Repository Implementations, External APIs, DB, Cache, Queue	Domain, Application
Presentation	Controllers, Actions, Request/Response, CLI	Application

Quick Checklists

Domain Layer Checklist

• No framework imports (Doctrine, Eloquent, Symfony)
• Entities have behavior, not just data
• Value Objects for domain concepts (Email, Money, Id)
• Repository INTERFACES defined here
• Enums for fixed value sets
• Domain Events for side effects
• No public function set*() methods

Application Layer Checklist

• UseCases orchestrate, don't decide
• DTOs for input/output
• No business logic (if/switch on domain state)
• Transaction boundaries here
• No HTTP/CLI concerns

Infrastructure Layer Checklist

• Implements Domain interfaces
• No business logic in repositories
• External service adapters
• Caching, queuing implementations

Presentation Layer Checklist

• Validates input
• Maps to DTOs
• Calls UseCase
• Formats response
• No business logic

Common Violations Quick Reference

Violation	Where to Look	Severity
use Doctrine\\ in Domain	Domain/*.php	Critical
use Illuminate\\ in Domain	Domain/*.php	Critical
use Infrastructure\\ in Domain	Domain/*.php	Critical
Only getters/setters in Entity	Domain/Entity/*.php	Warning
=== 'pending' magic strings	Any PHP file	Warning
public function set*()	Domain/Entity/*.php	Warning
Business logic in Controller	Presentation/*.php	Warning
Business logic in Repository	Infrastructure/*.php	Warning

PHP 8.4 DDD Patterns

Value Object

final readonly class Email
{
    public function __construct(
        public string $value
    ) {
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new InvalidArgumentException('Invalid email');
        }
    }

    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }
}

Entity with Behavior

final class Order
{
    private OrderStatus $status;

    public function __construct(
        private readonly OrderId $id,
        private readonly CustomerId $customerId
    ) {
        $this->status = OrderStatus::Pending;
    }

    public function confirm(): void
    {
        if (!$this->status->canTransitionTo(OrderStatus::Confirmed)) {
            throw new DomainException('Cannot confirm order');
        }
        $this->status = OrderStatus::Confirmed;
    }
}

Repository Interface

// Domain/Repository/OrderRepositoryInterface.php
interface OrderRepositoryInterface
{
    public function findById(OrderId $id): ?Order;
    public function save(Order $order): void;
}

References

For detailed information, load these reference files:

• references/layer-architecture.md — Detailed layer rules and boundaries
• references/domain-patterns.md — Entity, VO, Aggregate, Repository patterns
• references/application-patterns.md — UseCase, DTO, Command/Query patterns
• references/antipatterns.md — Common violations with detection patterns
• references/php-specific.md — PHP 8.4 specific implementations

Assets

• assets/report-template.md — Structured audit report template