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