CodeIgniter Knowledge Base

Quick reference for CodeIgniter 4 framework patterns and PHP implementation guidelines. CodeIgniter 4 is a complete rewrite of the framework, built for PHP 8.1+ with namespaces, autoloading, and a modern MVC architecture. It remains lightweight and fast while providing the tools needed for professional application development.

Core Principles

MVC Architecture

Request → index.php → Bootstrap → Routing
  → Filters (before) → Controller → Service/Model → View
  → Filters (after) → Response

Key principles:

• Lightweight core with minimal overhead
• HMVC-capable via modules (app/Modules/)
• Services container for dependency management
• Filters as middleware equivalent
• Entity classes for data hydration

Quick Checklists

DDD-Compatible CI4 Project

• Domain logic extracted from Models into Domain layer
• Repository interfaces defined in Domain, implemented in Infrastructure
• Value Objects used instead of primitive types for domain concepts
• CI4 Models used only as persistence adapters
• Services registered in Config\Services for dependency injection
• Business rules not in Controllers or Filters
• Domain Events dispatched through domain EventDispatcherInterface port (not CI4 Events directly)
• Input validation at Controller level (not in Domain)
• Shield Groups/Permissions used only at Filter/Route level; Domain uses Specifications
• Queue job handlers delegate to Application UseCases
• Infrastructure components (Cache, HTTP Client, Email) accessed via domain ports

Clean Architecture Checks

• No use CodeIgniter\... in Domain layer
• No direct database access outside Repository implementations
• Controllers delegate to Application Services / Use Cases
• DTOs used for cross-layer data transfer
• Config\Services wires interfaces to implementations
• Entities are framework-free domain objects (not CI4 Entity)

Common Violations Quick Reference

Violation	Where to Look	Severity
Business logic in Controller	app/Controllers/*.php	Critical
Domain depends on CI4 framework	use CodeIgniter\ in Domain classes	Critical
Direct DB queries in Controller	$this->db-> in Controllers	Critical
Model contains business rules	Complex if/switch in Model methods	Warning
Missing input validation	Controllers without $this->validate()	Warning
God Controller	Controllers > 300 lines	Warning
Shield UserIdentity in Domain	use CodeIgniter\Shield in Domain layer	Critical
Queue in Domain	service('queue') in Domain layer	Critical
Cache/HTTP Client in Domain	CacheInterface or CURLRequest in Domain	Critical
Missing queue resilience	Jobs without $retryAfter/$tries	Warning
Hardcoded config values	Magic strings instead of config()	Info

PHP 8.4 CodeIgniter Patterns

Controller (Thin, Delegates to Service)

<?php
declare(strict_types=1);
namespace App\Controllers\Api;

use App\Controllers\BaseController;
use CodeIgniter\HTTP\ResponseInterface;

final class OrderController extends BaseController
{
    public function __construct(
        private readonly OrderService $orderService,
    ) {}

    public function show(string $id): ResponseInterface
    {
        $order = $this->orderService->findById($id);
        return $this->response->setJSON(['id' => $order->id, 'status' => $order->status->value]);
    }
}

Service (Application Layer)

<?php
declare(strict_types=1);
namespace App\Services;

final readonly class OrderService
{
    public function __construct(
        private OrderRepositoryInterface $orders,
        private EventDispatcherInterface $events,
    ) {}

    public function confirm(string $orderId): void
    {
        $order = $this->orders->findById($orderId)
            ?? throw new OrderNotFoundException($orderId);
        $order->confirm();
        $this->orders->save($order);
        $this->events->dispatch(new OrderConfirmedEvent($order->id()));
    }
}

Model as Repository Adapter

<?php
declare(strict_types=1);
namespace App\Models;

use CodeIgniter\Model;

final class OrderModel extends Model
{
    protected $table         = 'orders';
    protected $primaryKey    = 'id';
    protected $returnType    = OrderEntity::class;
    protected $allowedFields = ['customer_id', 'status', 'total', 'notes'];
    protected $useTimestamps = true;
}

References

For detailed information, load these reference files:

• references/architecture.md -- CI4 MVC structure, modules, directory layout
• references/ddd-integration.md -- Domain extraction, Repository pattern, Value Objects in CI4
• references/routing-http.md -- Controllers, Filters, RESTful routing, validation
• references/persistence.md -- CI4 Model, Query Builder, Entity classes, migrations
• references/dependency-injection.md -- Services class, custom registration, DI patterns
• references/testing.md -- CIUnitTestCase, DatabaseTestTrait, feature testing, mocking
• references/security.md -- Shield authentication/authorization, Filters, CSRF, password hashing with DDD
• references/event-system.md -- CI4 Events, domain event dispatching, lifecycle events, testing
• references/queue.md -- codeigniter4/queue jobs, workers, retry, chained jobs, queue events with DDD
• references/infrastructure-components.md -- Cache, CURLRequest HTTP client, Email, Throttler with DDD ports
• references/antipatterns.md -- Common violations with detection patterns and fixes