CodeIgniter 3 to CodeIgniter 4 Migration Strategy

Mar 15, 2026

In 1880, the U.S. Census Bureau faced a monumental challenge. The previous census had taken eight years to complete—and with America’s rapid growth, the next one threatened to take even longer. Manual tabulation was reaching its limits. The solution came in the form of Herman Hollerith’s tabulating machine, which mechanized the counting process without changing the fundamental nature of census data. The technology didn’t just speed up existing work; it transformed what was possible—reducing a decade-long process to a single year.

Similarly, CodeIgniter 3 has served as a reliable workhorse for countless PHP applications, handling the mechanics of web development with pragmatic efficiency. But like the census clerks with their paper forms, developers working with CI3 have encountered growing limitations: the framework’s architecture predates modern PHP features, its autoloading feels dated compared to PSR-4 standards, and its design decisions reflect the PHP landscape of the mid-2010s rather than today. Moving from CI3 to CI4 isn’t merely an incremental upgrade — it’s a shift from mechanizing old workflows to embracing a fundamentally reimagined approach that aligns with contemporary PHP development.

CodeIgniter 4 represents a complete rewrite that adopts namespaces, implements modern autoloading, and restructures the framework around PHP 7.4+‘s capabilities. This article provides a strategic, phased approach to migration that acknowledges both the necessity of the transition and the practical realities of moving production applications. We’ll break the process into logical stages — each testable, each building toward a modern, maintainable codebase that will serve you well for years to come.

Why Migrate to CodeIgniter 4?

Before we dive into the “how,” let’s understand the “why.” The benefits of migrating go far beyond merely staying on a supported version. Of course, there are other paths you might consider—staying with CI3 and handling security updates yourself, or migrating to a different PHP framework like Laravel or Symfony entirely. Of course, there are other paths you might consider—staying with CI3 and handling security updates yourself, or migrating to a different PHP framework like Laravel or Symfony entirely. Each of these approaches has merit depending on your circumstances. However, if you have an existing CI3 codebase with moderate complexity and want to preserve your investment while modernizing, migrating to CI4 is often the most pragmatic choice.

Let’s examine what makes CI4 compelling:

One may wonder: Is this migration effort truly worth it, given that my application works fine in CI3? The answer depends on several factors—security, performance, and long-term maintainability chief among them. Consider that CI3 reached end-of-life in December 2021—no security patches have been released since. If you’re running a production application, especially one handling sensitive data, that’s a risk you cannot ignore indefinitely.

Modern PHP Support: CodeIgniter 4 requires PHP 7.4 or higher—this isn’t an arbitrary restriction; it enables you to use typed properties, arrow functions, null coalescing operators, and other features that improve code reliability and readability. The framework itself leverages these features throughout.
Enhanced Performance: CI4’s core has been streamlined and benchmarks show it can handle requests with significantly less memory usage than CI3—real-world applications often report 20-30% performance improvements.
Improved Security: Beyond basic protections, CI4 introduces Content Security Policy (CSP) configuration, enhanced XSS filtering, and better handling of cross-site request forgery. The framework’s security team actively addresses vulnerabilities—unlike CI3, which reached end-of-life in December 2021.
Namespaces and Autoloading: The shift to PSR-4 autoloading eliminates the fragile custom autoloader that CI3 required. This means you get Composer’s reliable class loading—a battle-tested standard used across the PHP ecosystem.
Powerful New Features: A built-in CLI tool (spark), a more flexible routing system with HTTP verb restrictions, built-in validation and encryption libraries, and a layered architecture that supports modular development—CI4 gives you tools that modern PHP applications expect.

Considerations and Alternatives

Of course, migrating to CI4 isn’t the only path forward. Let’s briefly examine the other options you might consider:

Option 1: Stay on CI3 with Extended Support Some organizations choose to keep their CI3 applications running indefinitely. This is feasible if your application is stable, internal, and not exposed to internet-facing security risks. However, you’ll need to shoulder the burden of backporting security patches yourself—or rely on community-maintained forks that may or may not be comprehensive. For most production applications, especially those handling sensitive data, this carries unacceptable risk.

Option 2: Migrate to a Different Framework Entirely Laravel, Symfony, or Yii are all mature PHP frameworks with active ecosystems. A full framework migration—rebuilding your application on a new foundation—offers the cleanest architectural break. However, this approach typically takes 2-5 times longer than a direct CI3-to-CI4 migration, requires comprehensive rewrite of business logic, and demands your team learn an entirely new framework. This makes sense if you’re already planning a major refactor or if your application needs capabilities CI4 doesn’t provide.

Option 3: Incremental Coexistence (Advanced) For very large applications, you might run CI3 and CI4 side-by-side, gradually migrating modules. This approach—while technically possible—requires sophisticated routing configuration and careful session management. Few teams attempt it, and we won’t cover it in detail here—though, of course, if you have a massive application with a team of experienced CI developers, it’s worth exploring. Our phased approach assumes a direct migration where the application is offline or running in maintenance mode during the final cutover. Of course, we don’t need to discuss exhaustive edge cases in this guide; our focus is on the path most teams will take.

Given these options—and assuming you want to preserve your CI3 investment while modernizing—the direct migration path we outline below is typically the most pragmatic choice.

Key Differences Between CI3 and CI4

A successful migration starts with understanding the fundamental architectural shifts between the two versions.

Feature	CodeIgniter 3	CodeIgniter 4
PHP Version	PHP 5.6+	PHP 7.4+
Directory	`application`, `system`, `public` at root	`app`, `system`, `public`, `writable` (more organized)
Namespaces	Not used	Fully implemented (PSR-4)
Routing	Convention-based, in `application/config/routes.php`	Configuration-based, in `app/Config/Routes.php` (more explicit)
Controllers	Extend `CI_Controller`	Extend `CodeIgniter\Controller`
Models	Extend `CI_Model`, often procedural logic	Extend `CodeIgniter\Model` with built-in CRUD, plus Entities
Libraries/Helpers	Loaded via `$this->load`	Autoloaded via namespaces or explicitly configured

The Migration Strategy: A Phased Approach

Instead of a “big bang” migration, we recommend a phased approach. This allows you to tackle the migration in logical chunks, making it easier to test and debug along the way.

Phase 1: Preparation and Planning

This is the most critical phase—good preparation will save you countless hours of debugging later. We’ll start by assessing what we have, then set up our migration workspace.

First, let’s perform a systematic code audit. We need to review the entire CI3 application and catalog: third-party libraries (in application/third_party), custom helpers, any modifications to the core system files, and Composer dependencies. For each item, ask: Is this still needed? Is there a CI4-compatible replacement? Does this functionality exist natively in CI4? One may wonder: what if a critical library has no CI4 version? We’ll need to either find an alternative or create a wrapper—we’ll cover that in Phase 2. Of course, if your application is relatively small with few dependencies, this process will go much more quickly.

Next, check your composer.json (if present). Ensure all dependencies specify compatibility with PHP 7.4 or higher. You might be surprised to find some older libraries that haven’t been updated in years—these will need attention.

Now, set up a fresh CI4 project. Do not try to upgrade your CI3 application in place; the directory structures are fundamentally different. Instead, create a new project side-by-side with your existing code:

composer create-project codeigniter4/appstarter my_ci4_project

When you run this command, you’ll see output similar to:

Creating a new CodeIgniter 4 project in /path/to/my_ci4_project
  - Installing codeigniter4/appstarter (4.4.5)
  - Installing codeigniter4/framework (4.4.5)
  ...snip...
  - Optimizing Composer autoloader...
  - Writing compiled file to /path/to/my_ci4_project/vendor/composer/autoload_real.php
  - Generating autoload files

Before running this command—or any command that modifies your filesystem—ensure your CI3 project is committed to version control. This safety net means you can always revert if something goes awry.

This gives us a clean foundation — we’ll gradually move code into this new structure.

Finally — though it should go without saying — ensure your CI3 project is under version control. Create a dedicated branch for the migration work. We recommend naming it something like migrate-to-ci4 so the purpose is clear. This branch becomes our safety net; if we need to revert or compare, we have a pristine record of the original code.

Phase 2: Migrating Libraries and Helpers

Now that we have our workspace set up—and assuming we’ve completed our audit in Phase 1—we’re ready to start moving code. Let’s start with the low-hanging fruit: custom code that doesn’t heavily depend on CI3’s core. This is a good place to build momentum—we’ll see quick wins while learning the new structure. Of course, if you discover during this phase that a library requires extensive refactoring, you might choose to defer it to later phases; that’s perfectly reasonable.

In CI3, helpers are procedural functions—just PHP functions loaded globally. Libraries are classes typically loaded via $this->load->library(). In CI4, both are autoloaded via namespaces, but the directory structure and naming conventions differ.

Helpers: Move your application/helpers files to app/Helpers. You’ll need to rename them following CI4’s convention: my_helper.php instead of my_helper_helper.php is acceptable—but make sure any function calls in your code match. The good news: helpers work identically; they’re just autoloaded automatically when placed in the right directory.

Libraries: Move files from application/libraries to app/Libraries. Then, we need to refactor for namespaces. Let’s walk through a concrete example—imagine we have a Email_notification library that sends templated emails. Here’s the CI3 version:

We could copy this file directly—but that would miss the fundamental architectural shift between CI3 and CI4. The key change is that we no longer access the CI super object through get_instance(); instead, we use CI4’s service container. Let’s see how.

Walkthrough: Migrating a Library Step-by-Step

Let’s walk through the entire migration process for this library, including the actual commands we’d run and what we’d see.

Step 1: Copy the file

First, we copy our library from the CI3 project:

cp application/libraries/Email_notification.php my_ci4_project/app/Libraries/

Step 2: Rename and refactor

Now we need to rename the file to follow CI4 conventions and refactor the code. CI4 uses PascalCase for class names; we’ll also add the namespace. Here’s what we’ll change:

mv my_ci4_project/app/Libraries/Email_notification.php my_ci4_project/app/Libraries/EmailNotification.php

Then we edit the file, replacing the contents with the CI4 version we showed earlier. Notice we removed the defined('BASEPATH') check—it’s no longer needed. We added namespace App\Libraries; at the top, and we’re using service('email') instead of get_instance().

Step 3: Verify the refactoring

We should verify our changes work. One way is to create a simple test script:

// my_ci4_project/app/Controllers/Test.php
<?php

namespace App\Controllers;

use CodeIgniter\Controller;

class Test extends Controller
{
    public function index()
    {
        $notifier = new \App\Libraries\EmailNotification();
        $result = $notifier->sendWelcome('test@example.com', 'Test User');
        
        echo $result ? 'Email sent!' : 'Email failed.';
    }
}

Then we visit http://localhost:8080/test to verify it works.

Of course, this is a relatively simple library—your libraries may require more extensive refactoring, especially if they interact deeply with CI3’s super object. The pattern remains the same: replace $this->CI->load patterns with CI4’s service calls, remove get_instance(), and add proper namespaces.

CI3 Library (application/libraries/Email_notification.php):

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Email_notification {
    protected $CI;
    
    public function __construct() {
        $this->CI =& get_instance();
        $this->CI->load->library('email');
    }
    
    public function send_welcome($user_email, $user_name) {
        $this->CI->email->from('noreply@example.com', 'Our App');
        $this->CI->email->to($user_email);
        $this->CI->email->subject('Welcome!');
        $this->CI->email->message("Hello $user_name, welcome to our app!");
        return $this->CI->email->send();
    }
}

To migrate this to CI4, we’ll need to:

Add the namespace App\Libraries
Remove the defined('BASEPATH') check
Remove the $this->CI =& get_instance() pattern—we’ll use dependency injection or service location instead
Update method names to camelCase (optional but recommended)

Here’s the migrated version:

CI4 Library (app/Libraries/EmailNotification.php):

<?php

namespace App\Libraries;

use CodeIgniter\Email\Email;

class EmailNotification
{
    protected Email $email;
    
    public function __construct()
    {
        $this->email = service('email'); // Use CI4's service() helper
    }
    
    public function sendWelcome(string $userEmail, string $userName): bool
    {
        $this->email->setFrom('noreply@example.com', 'Our App');
        $this->email->setTo($userEmail);
        $this->email->setSubject('Welcome!');
        $this->email->setMessage("Hello $userName, welcome to our app!");
        return $this->email->send();
    }
}

Notice the differences: we use service('email') to get the email service—no more get_instance(). We’ve also added type hints (optional but encouraged in CI4). The class name uses upper camel case instead of snake case. And crucially — the file is in app/Libraries with the appropriate namespace at the top.

Now in your controller, you would use this library like:

$notifier = new \App\Libraries\EmailNotification();
$notifier->sendWelcome($user->email, $user->name);

Or, if you prefer to inject it through the constructor (which makes testing easier), that’s supported as well. For now — though — the above approach will get you started quickly.

Of course, this example is relatively simple. If your CI3 libraries make heavy use of $this->db or other loaded services, you’ll need to adapt them to use CI4’s service container. The principle, though, remains the same: remove the old CI instance pattern, add namespaces, and use the new service locations.

Phase 3: Migrating Models

Models undergo perhaps the most significant transformation in the CI3-to-CI4 migration. CI3 models are typically procedural—you call $this->db methods directly and return raw arrays. CI4 models extend a base Model class that provides built-in CRUD operations, automatic validation, and optional Entity integration for richer data mapping.

One may wonder: Why such a dramatic change? The answer lies in CI4’s embrace of modern PHP patterns—specifically, the Active Record pattern with more structure and type safety. Let’s walk through a typical migration. Suppose we have a Product_model in CI3 that handles inventory:

CI3 Model (application/models/Product_model.php):

<?php
class Product_model extends CI_Model {
    public function get_active_products($category = null) {
        $this->db->where('active', 1);
        if ($category) {
            $this->db->where('category', $category);
        }
        $query = $this->db->order_by('name', 'ASC')->get('products');
        return $query->result_array();
    }
    
    public function update_stock($product_id, $quantity) {
        $data = ['stock' => $quantity];
        $this->db->where('id', $product_id);
        return $this->db->update('products', $data);
    }
}

This pattern—manual query building, returning arrays—is standard in CI3. To migrate this to CI4, we’ll move it to app/Models/ProductModel.php, add the namespace, and leverage CI4’s Model features:

CI4 Model (app/Models/ProductModel.php):

<?php

namespace App\Models;

use CodeIgniter\Model;

class ProductModel extends Model
{
    protected $table = 'products';
    protected $primaryKey = 'id';
    protected $allowedFields = ['name', 'category', 'stock', 'active'];
    protected $useTimestamps = true; // Automatically manage created_at, updated_at
    protected $returnType = 'array'; // Could also be 'object' or an Entity class
    
    /**
     * Get active products, optionally filtered by category
     */
    public function getActiveProducts($category = null)
    {
        $builder = $this->builder();
        $builder->where('active', 1);
        
        if ($category) {
            $builder->where('category', $category);
        }
        
        return $builder->orderBy('name', 'ASC')->findAll();
    }
    
    /**
     * Update product stock
     */
    public function updateStock(int $product_id, int $quantity): bool
    {
        return $this->update($product_id, ['stock' => $quantity]);
    }
}

Notice the improvements: we’ve replaced the manual $this->db queries with CI4’s builder methods. But we’ve also gained automatic timestamp handling, clearer method names, and—perhaps most importantly—we can now use the built-in find methods like $this->find($id) or $this->where()->findAll() without writing query code at all. Of course, this means we need to be mindful of mass assignment vulnerabilities, which is why defining $allowedFields is mandatory—CI3 had no such protection.

One may wonder: what about Entities? If you want domain logic on your data objects—methods like $product->isInStock() or $product->getPriceWithTax()—create an Entity class. Here’s how:

Entity (app/Entities/Product.php):

<?php

namespace App\Entities;

use CodeIgniter\Entity\Entity;

class Product extends Entity
{
    /**
     * Example computed property
     */
    public function isInStock(): bool
    {
        return isset($this->stock) && $this->stock > 0;
    }
    
    /**
     * Example mutator - runs when setting a value
     */
    public function setPrice($value)
    {
        $this->attributes['price'] = number_format($value, 2);
    }
}

Then in your model, set protected $returnType = 'App\Entities\Product'; and you’ll get entity objects instead of plain arrays. Entities let you add business logic to your data—something CI3’s bare arrays couldn’t do elegantly.

Of course, not every model needs an Entity — for simple data transfer, arrays work fine. But for complex domain logic, Entities provide a clean separation of concerns.

Now, you can update your controllers. Where you previously had:

$data['products'] = $this->product_model->get_active_products('electronics');

You now write:

$data['products'] = $this->productModel->getActiveProducts('electronics');

Notice the camelCase method names—this is a CI4 convention, though not strictly required. We recommend following it for consistency with the framework.

Phase 4: Migrating Controllers and Routes

This phase—routing and controllers—typically requires the most manual effort. CI3’s convention-based routing (where example.com/users/show/5 automatically maps to Users controller, show method, with 5 as parameter) is replaced by CI4’s explicit configuration. Every route must be declared in app/Config/Routes.php. This change offers more control and clarity—though it means we can’t simply copy controllers over without also defining their routes.

One may wonder: Why did CI4 abandon the convention-based routing that made CI3 so easy to get started with? The answer is control. As applications grow, convention-based routes become ambiguous—what should example.com/admin/users map to? An Admin controller? A Users controller with an admin prefix? CI4’s explicit routing eliminates that guesswork, making your application’s intent crystal clear to anyone reading the code.

Let’s start with a complete walkthrough. Suppose we have a CI3 Products controller:

CI3 Controller (application/controllers/Products.php):

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Products extends CI_Controller {
    public function __construct() {
        parent::__construct();
        $this->load->model('product_model');
        $this->load->library('form_validation');
    }
    
    public function index($category = null) {
        $data['products'] = $this->product_model->get_active_products($category);
        $this->load->view('products/list', $data);
    }
    
    public function show($id) {
        $data['product'] = $this->product_model->find($id);
        if (!$data['product']) {
            show_404();
        }
        $this->load->view('products/detail', $data);
    }
    
    public function create() {
        $this->form_validation->set_rules('name', 'Name', 'required');
        $this->form_validation->set_rules('price', 'Price', 'required|numeric');
        
        if ($this->form_validation->run() === FALSE) {
            $this->load->view('products/create');
        } else {
            $this->product_model->insert($_POST);
            redirect('products');
        }
    }
}

In CI3, this controller would automatically handle URLs like /products, /products/show/5, and /products/create based on convention. In CI4, we need to both create the new controller and define routes.

CI4 Controller (app/Controllers/Products.php):

<?php

namespace App\Controllers;

use CodeIgniter\Controller;

class Products extends BaseController
{
    protected $productModel;
    
    public function initController(\CodeIgniter\HTTP\RequestInterface $request, \CodeIgniter\HTTP\ResponseInterface $response, \Psr\Log\LoggerInterface $logger)
    {
        parent::initController($request, $response, $logger);
        
        $this->productModel = model('ProductModel'); // CI4's model helper
    }
    
    public function index($category = null)
    {
        $data['products'] = $this->productModel->getActiveProducts($category);
        return view('products/list', $data);
    }
    
    public function show($id)
    {
        $data['product'] = $this->productModel->find($id);
        
        if (!$data['product']) {
            throw \CodeIgniter\Exceptions\PageNotFoundException::forPageNotFound();
        }
        
        return view('products/detail', $data);
    }
    
    public function create()
    {
        helper(['form', 'url']);
        
        if (! $this->validate([
            'name' => 'required',
            'price' => 'required|numeric'
        ])) {
            return view('products/create');
        }
        
        $this->productModel->insert([
            'name' => $this->request->getPost('name'),
            'price' => $this->request->getPost('price'),
        ]);
        
        return redirect()->to('/products');
    }
}

Key changes:

Namespace App\Controllers added
Extends BaseController (or Controller), not CI_Controller
No constructor dependency injection via $this->load; instead we use model() helper or direct new \App\Models\ProductModel()
Input accessed via $this->request->getPost() or $this->request->getVar()
Validation uses $this->validate() method, returning boolean
Views returned with return view() rather than $this->load->view()
404 errors throw PageNotFoundException
Redirects use helper function redirect()->to()

Notice the pattern: CI4 favors explicit service location—we call model() directly or instantiate classes ourselves. CI3’s $this->load->model() was more implicit. This change makes dependencies clearer, though it requires adjusting to a new workflow.

Now, we must define routes. Open app/Config/Routes.php (created during CI4 installation) and add:

CI4 Routes (app/Config/Routes.php):

<?php

$routes = Services::routes();

// Define the default route
$routes->get('/', 'Home::index');

// Products routes
$routes->get('products', 'Products::index');
$routes->get('products/category/(:any)', 'Products::index/$1');
$routes->get('products/show/(:num)', 'Products::show/$1');
$routes->match(['get', 'post'], 'products/create', 'Products::create');
$routes->post('products', 'Products::store'); // For RESTful design

/*
 * --------------------------------------------------------------------
 * Additional Route Patterns
 * --------------------------------------------------------------------
 */

Notice we explicitly defined each URL pattern. CI4 supports match() for multiple HTTP verbs—here we use it for create which displays the form on GET and processes it on POST. Many developers prefer fully RESTful routes (separate store method for POST). Choose what fits your style.

Also note: CI4 uses (:any) for any characters except forward slash, and (:num) for numeric only—similar to CI3’s :any and :num but with slightly different syntax. The parameter is passed to your method as an argument.

One important difference: in CI3, $this->input->post('field') retrieved sanitized input. In CI4, $this->request->getPost('field') gives you raw input. For filtering, use the $this->request->getPost('field', FILTER_SANITIZE_STRING) overload or apply validation rules—the latter is preferred.

Of course, this example covers only the basics — CI4 routing supports route filters (similar to middleware), route groups, and automatic controller discovery for REST resources. We’ll stick to fundamentals here, but as you grow comfortable, explore app/Config/Filters.php for authentication and CORS handling.

Phase 5: Updating Views

Views typically require the least modification—though this doesn’t mean we can copy them blindly. In CI3, views could access the CI super object via $this and call loaded libraries directly. This no longer works in CI4; views receive only the data we explicitly pass to them. Let’s walk through this systematically.

Here’s our approach:

Copy all files from application/views to app/Views.
Search for any $this-> references inside views. In CI3, you might have $this->session->userdata('name') or $this->config->item('base_url'). In CI4, you’ll need to either:
- Pass those values from the controller as part of the $data array, or
- Use the global helper functions: session('name'), config('App')->baseURL
Check for any usage of $this->load->view() within other views—view composition still works, but the syntax is now return view('partials/header', $data); in controllers, and within views you can still use <?= view('partials/header') ?>.
Review any embedded PHP short tags <?=—these work fine in modern PHP, but ensure your views don’t have <? short open tags (these may be disabled on some servers).
Finally, check any HTML forms. CI3 used form_open() helpers that automatically added CSRF tokens. CI4’s form helper still exists but CSRF protection is now managed by the Security filter—verify your forms are protected either through filters or manual token inclusion. Of course, you may wonder why CSRF handling changed; it’s because CI4 moved to a filter-based middleware system that’s more flexible and explicit.

Phase 6: Testing and Deployment

Testing is not the final step—it’s an ongoing process throughout migration. That said, we typically perform system-wide testing after all components are in place.

First, let’s verify our CI4 installation has testing capabilities enabled. By default, the appstarter package includes PHPUnit. Check phpunit.xml.dist exists at your project root. If not, you may need to install PHPUnit via Composer: composer require --dev phpunit/phpunit. Running this command will show output like:

Using version ^10.0 for phpunit/phpunit
...
  - Installing phpunit/phpunit (10.5.10)
  ...snip...
  - Generating autoload files

Now, run the built-in test suite that CI4 provides:

php spark test

When you run this, you’ll see something like:

PHPUnit 10.5.10 by Sebastian Bergmann and contributors.

............................................  36 / 1140 (  3%)
...snip...

Time: 00:00.123, Memory: 12.00 MB

OK (40 tests, 143 assertions)

This runs the framework’s own tests to ensure your installation is sound. Of course, we also need to test our migrated application. Write feature tests for critical paths—especially those areas where behavior might differ between CI3 and CI4. For example:

Routing: Does /users/5 correctly route to Users::show(5)?
Input retrieval: Are you using $this->request->getPost() correctly?
Model operations: Do find(), insert(), and update() work as expected?
Session persistence: Does user login state survive across requests?

CI4’s testing framework lets you simulate HTTP requests:

public function testProductListLoads()
{
    $result = $this->get('products');
    $result->assertStatus(200);
    $result->assertSee('Product List');
}

Running these tests in a staging environment—where PHP version, database, and server configuration match production—gives us confidence before deployment.

Once tests pass, deploy to staging first — never skip staging. Use the same deployment process you’ll use in production (Git hooks, Capistrano, etc.). Test all critical user journeys. Have your team perform exploratory testing. Check logs for warnings or notices that may indicate misconfiguration.

Before you even reach staging, though, we should address database changes—one may wonder: What happens if my migration requires schema modifications? This deserves its own warning: always test database migrations on a copy of production data first. A botched schema change can render your application unusable. You’ll need a database migration strategy—perhaps using CI4’s built-in migration commands (php spark migrate) or a tool like Phinx. Of course, if you’re using CI3’s migrations already, you’ll need to convert those to CI4’s format.

When staging validates successfully, plan the production cutover. Consider:

Session continuity: If users are logged in during the cutover, will sessions survive? You may need to clear sessions or implement a shared session store.
Rollback plan: What if something goes wrong? Have your staging environment ready to revert to the old version quickly.

Finally, deploy to production during low-traffic hours. Monitor error logs closely for 24-48 hours. Be prepared to roll back if critical issues arise.

Common Challenges and Solutions

While every migration is unique, we’ve observed recurring challenges:

Third-Party Libraries: If your CI3 app depends on libraries without CI4 versions, you have options: find modern alternatives (often the best path), create wrapper classes that adapt the old library to CI4’s patterns, or—if feasible—replace functionality with CI4’s built-in libraries. For instance, many CI3 applications used Ion Auth for authentication; CI4’s built-in authentication libraries (or community packages like codeigniter4/auth) may suffice.

index.php Removal: CI4 makes this straightforward. The public/.htaccess file (included by default) handles URL rewriting. Just ensure your web server’s document root points to the public directory—you should never serve your application from the project root. If you must keep index.php, adjust app/Config/App.php $indexPage setting accordingly.

Session Handling: CI4’s session library is fundamentally different from CI3’s. It supports multiple drivers (files, database, Redis) and better security. If you store sessions in a database, you’ll need to either:

Migrate session data manually (export old ci_sessions table and import to new ci_sessions table—note schema differences)
Or force all users to log in again after deployment (often acceptable for small applications).

Configuration Migration: Your application/config/*.php files need to move to app/Config/. Pay special attention to config.php (base URL, encryption key), database.php, and autoload.php. CI4’s config system is namespaced—your config('App')->baseURL replaces $this->config->item('base_url'). Also note CI4’s environment-specific configuration: you can have .env files for local settings.

Helper Function Changes: Some helper functions have been renamed or replaced. For example, anchor() still exists but some URL helpers changed. The best approach: run your test suite, note failing calls, and check CI4’s documentation for the current helper equivalents.

CSRF Protection: CI4 enables CSRF protection by default in app/Config/Filters.php. Verify that your forms include CSRF tokens—if you used form_open() in CI3, the token was automatic. In CI4, you’ll need to add <input type="hidden" name="<?= csrf_token() ?>" value="<?= csrf_hash() ?>" /> manually or use the form helper csrf_field().

Of course, these aren’t the only challenges — you’ll encounter edge cases unique to your application. The key is to test thoroughly and consult CI4’s migration guide (available on the official CodeIgniter website) for a complete list of breaking changes.

Conclusion

We’ve covered a lot of ground—from the initial rationale for migration, through the six phases of transformation, to the common challenges you’ll likely encounter. Let’s return to our opening metaphor. Remember Hollerith’s tabulating machine? It didn’t just count faster—it changed what was possible with census data. Similarly, CodeIgniter 4 doesn’t just modernize your application; it fundamentally expands what you can do within the framework: modern PHP features, better performance, a thriving ecosystem, and a security team actively addressing vulnerabilities.

Migrating from CodeIgniter 3 to CodeIgniter 4 is a significant undertaking—there’s no sugar-coating that. But it’s a necessary investment. Your application will be more secure, more performant, and more maintainable. And—perhaps most importantly—you’ll be positioned to take advantage of PHP’s continuing evolution.

Of course, the migration guide we’ve provided covers the most common scenarios. However, every application is unique. If you encounter situations not covered here—complex third-party integrations, massive databases requiring zero-downtime migrations, or heavily customized core modifications—consider seeking assistance from developers experienced in CI4 migrations or consulting the official CodeIgniter 4 documentation. We don’t pretend this guide addresses every possible edge case—but it gives you a solid foundation.

Our phased approach—preparation, libraries, models, controllers/routes, views, then testing—lets you progress methodically. We recommend starting with a non-critical application if you have one; the lessons learned will smooth your production migration. Though, if you must migrate a production application immediately, our approach still works—just allocate more time for the unknown.

We wish you success with your migration. The CodeIgniter community remains active, and the framework’s continued evolution ensures it will serve PHP developers well into the future.

Tip: Keep your CI3 code accessible during migration by using version control branches. You’ll be surprised how often you need to reference the original implementation for edge cases or business logic details that weren’t initially documented.