Upgrade From Version 2.1.3 To Version 3.1 or 4

Hello, version 2.1.3 wont work on PHP 7, as for the upgrade, it is easier to upgrade to 3 (not that radical of a change) but i would recommend version 4.

It may not even be that much work, pretty much the "top" of the files (Controllers, Models, etc) changes (check provided examples on downloaded framework).
The calling of methods (for databases for example) change from like:
- $this->db->order_by("x"); to $this->db->orderBy("x");
- $this->input->post is now $this->request->getPost
- $this->load->model('model_x') -> $model = new \App\Model\Model_x();
- etc
You can probably do most of it with a Find and Replace (just make sure you re-check).

Thanks for your prompt reply Eskiso and as I mentioned we only want to keep this application for another 2-3 years and no more new development therefore it's not cost effective to rewrite the whole application in CodeIgniter 4. If CodeIgniter 3 upgrade process is simpler with minimal changes and will work with latest PHP7 and MySQL then will go with it. The only think not clear is do I have to go through multiple upgrade steps (e.g. From 2.1.3 to 2.1.4, next 2.1.4 to 2.2.x and then 2.2.x to 3.x.x) or I can directly upgrade from 2.1.3 to 3.x.x.

Thanks,
SauGo