Using CI version 3.1.3

Need to work on a an existing 5 year old CodeIgniter ver 3.1.3 project.
1. What's the difficulty level in upgrading to 4.3.1 ?
2. Any chance of using InertiaJS for CI3 ?

CodeIgniter 3 to 4 Upgrade Helper

By @kenjis

What did you Try? What did you Get? What did you Expect?

Joined CodeIgniter Community 2009.  ( Skype: insitfx )

Thanks you for the link.

The latest version of codeigniter is supported only on php 5.3.7+, so I highly advice you to upgrade your PHP version.

You could downgrade your project to codeigniter 3.0.5, which is the last version that supports php 5.2.x, but it's not recommended.

Upgrading from CodeIgniter version 3.1.3 to version 4.3.1 can be challenging, as there have been significant changes made to the framework. CodeIgniter 4 introduced a more modular architecture, updated its folder structure, and made changes to several libraries and features, including URL routing and caching. As a result, updating an existing project can be time-consuming, and some functionality may need to be rewritten or updated to work with the new version.
Before upgrading, it is essential to thoroughly test your code to ensure that it still functions correctly. It is also recommended to create a backup of your project files and database before making any changes to avoid data loss.

InertiaJS is a client-side framework that allows developers to build modern, reactive web applications without sacrificing the simplicity and elegance of server-side routing. While InertiaJS is designed to work with Laravel and Vue.js out of the box, it can be used with other PHP frameworks such as CodeIgniter, albeit with some extra work.
To use InertiaJS with CodeIgniter 3, you would need to manually integrate the Inertia middleware and create routes that map to the Inertia controller. This involves modifying the application's routing, creating controllers and views that work with InertiaJS, and potentially updating some of the project's dependencies.

While it is technically possible to use InertiaJS with CodeIgniter 3, it may be easier to use a different client-side framework or library that is more compatible with your existing codebase. Alternatively, you could consider upgrading to CodeIgniter 4, which includes built-in support for using InertiaJS with its new view layer, Spark.