3 generation referral system in php

I need the source code of 3 generation earn referral system in php, this is how i want it to work.

1. A user signs up with a fee of 2K.
2. He makes his first referral which will be his 1st generation, then he earns 50% from the sign up fee which is 1K
3. Then his 1st generation now went ahead to refer another person which will be his 2nd generation, the 1st generation will then get 50% from the 2nd generation.
4. when the 2nd generation person refers someone which will be in the 3rd generation now, the 2nd generation will now earn 50% from the 3rd generation also.
   Same thing keeps repeating on and on in the system.