I would humbly to know how to create and develop a referral program for online business websites and apps

Question

It's give one opportunity to invite a friend and get rewards

Answer 1

Make the referral code the same as their username, then you won't have to worry about avoiding duplicate codes and it makes it very clear which account it's being associated with.

For the rest of it, you'll simply store stuff in your database and reference it. For example, when the friend signs up they'll enter the other person's referral code (username) and that'll be stored in their database record. The referral code will provide a means of easily associating the friend's account with the one who referred them. Then just write code that'll check for any conditions the referral programs require, and upon meeting those conditions, update the two accounts with whatever reward they get.

In regards to only having one invite, simply store the person they invited as an entry in their database record. Then you can simply check to see if that field has a value or not; if it does then they can't invite another person and if it doesn't, they can.