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

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

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