Create custom link url for users

I have a membership site where people can sign up for an account using their name, phone number and password of choice. The information entered during registration is stored in a MySQL database table called 'users'. The primary key in the users table is the phone number. During sign up, the user has the option to type the phone number of his referrer into 'referrer' field.
I have all these set up and working fine, but I want to enhance the site some more, by:

1. Creating a unique referral link for users which will have their phone number appended to their link. I want to track which user referred others to my site so I can reward them.
   
2. When they share this link, and people click on it, it will take them to the registration page (register.php). On that page he phone number of the referrer will be set (and locked) in the 'referrer' box.
   

I am new to php and Code Igniter. I have toured the internet looking for a solution to this but i just can't find any. Please any help will be much appreciated.