How to convert this code from Core PHP to Codeigniter ?

I think that you must understand PHP, SQL and Codeigniter to succeed with your project unless you are willing to hire coders to do it.

So, if you are willing to learn, I recommend this:
Download the framework. Then follow the manual starting with the setup. Also, https://avenir.ro/codeigniter-tutorials/ was a great help when I started using Codeigniter. 

After the setup, you will find the answer to many (problary not all) of your questions when you reach this section: https://codeigniter.com/user_guide/tutor...ction.html

Good luck!