Upgrading from 3.x to 4.x

CodeIgniter 4 is a rewrite of the framework and is not backwards compatible. It is more appropriate to think of converting your app, rather than upgrading it. Once you have done that, upgrading from one version of CodeIgniter 4 to the next will be straightforward.

The “lean, mean and simple” philosophy has been retained, but the implementation has a lot of differences, compared to CodeIgniter 3.

There is no 12-step checklist for upgrading. Instead, start with a copy of CodeIgniter 4 in a new project folder, however you wish to install and use it, and then convert and integrate your app components. We’ll try to point out the most important considerations here.

To upgrade your project, we figured out two major tasks you have to work on. First of all, there are some general adjustments which are significant to every project and have to be handled. The second one are the libraries in which CodeIgniter is built up and contain some of the most important functions. These libraries operate separately from each other, so you have to look at them one by one.

Do read the user guide before embarking on a project conversion!

General Adjustments
Upgrading Libraries

General Adjustments 

Downloads 

CI4 is still available as a ready-to-run zip or tarball.
It can also be installed using Composer.

Namespaces 

CI4 is built for PHP 7.4+, and everything in the framework is namespaced, except for the helpers.

Application Structure 

The application folder is renamed as app and the framework still has system folders, with the same interpretation as before.
The framework now provides for a public folder, intended as the document root for your app.
There is also a writable folder, to hold cache data, logs, and session data.
The app folder looks very similar to application for CI3, with some name changes, and some subfolders moved to the writable folder.
There is no longer a nested application/core folder, as we have a different mechanism for extending framework components (see below).

Routing 

The Auto Routing is disabled by default. If you want to use the Auto Routing in the same way as CI3, you need to enable Auto Routing (Legacy).
CI4 also has an optional new more secure Auto Routing (Improved).

Model, View and Controller 

CodeIgniter is based on the MVC concept. Thus, the changes on the Model, View and Controller are one of the most important things you have to handle.
In CodeIgniter 4, models are now located in app/Models and you have to add the lines namespace App\Models; along with use CodeIgniter\Model; right after the opening php tag. The last step is to replace extends CI_Model with extends Model.
The views of CodeIgniter 4 have been moved to app/Views. Furthermore, you have to change the syntax of loading views from $this->load->view('directory_name/file_name') to echo view('directory_name/file_name');.
Controllers of CodeIgniter 4 have to be moved to app/Controllers. After that, add namespace App\Controllers; after the opening php tag. Lastly, replace extends CI_Controller with extends BaseController.
For more information we recommend you the following upgrade guides, which will give you some step-by-step instructions to convert the MVC classes in CodeIgniter4:

Class Loading 

There is no longer a CodeIgniter “superobject”, with framework component references magically injected as properties of your controller.
Classes are instantiated where needed, and components are managed by Services.
The class loader automatically handles PSR-4 style class locating, within the App (app) and CodeIgniter (i.e., system) top level namespaces; with Composer autoloading support.
You can configure the class loading to support whatever application structure you are most comfortable with, including the “HMVC” style.

Libraries 

Your app classes can still go inside app/Libraries, but they don’t have to.
Instead of CI3’s $this->load->library(x); you can now use $this->x = new X();, following namespaced conventions for your component.

Helpers 

Helpers are pretty much the same as before, though some have been simplified.
In CI4, redirect() returns a RedirectResponse instance instead of redirecting and terminating script execution. You must return it.
- redirect() Documentation CodeIgniter 3.X
- redirect() Documentation CodeIgniter 4.X

Events 

Hooks have been replaced by Events.
Instead of CI3’s $hook['post_controller_constructor'] you now use Events::on('post_controller_constructor', ['MyClass', 'MyFunction']);, with the namespace CodeIgniter\Events\Events;.
Events are always enabled, and are available globally.

Extending the Framework 

You don’t need a core folder to hold MY_... framework component extensions or replacements.
You don’t need MY_x classes inside your libraries folder to extend or replace CI4 pieces.
Make any such classes where you like, and add appropriate service methods in app/Config/Services.php to load your components instead of the default ones.

Upgrading Libraries 

Your app classes can still go inside app/Libraries, but they don’t have to.
Instead of CI3’s $this->load->library(x); you can now use $this->x = new X();, following namespaced conventions for your component.
Some libraries from CodeIgniter 3 no longer exists in Version 4. For all these libraries, you have to find a new way to implement your functions. These libraries are Calendaring, FTP, Javascript, Shopping Cart, Trackback, XML-RPC /-Server, and Zip Encoding.
CI3’s Input corresponds to CI4’s IncomingRequest.
CI3’s Output corresponds to CI4’s Responses.
All the other libraries, which exist in both CodeIgniter versions, can be upgraded with some adjustments. The most important and mostly used libraries received an Upgrade Guide, which will help you with simple steps and examples to adjust your code.

Note

More upgrade guides coming soon