How to Generate Unique ID based on First Letter of Firstname, Lastname and random number?

Question

Im using GravityForm & WordPress. My registration form have basic detail of registration such as firstname, lastname, email and password.

I would like to create unique id based on first letter of firstname and lastname then random number 2 digits only after it. Sample:

Firstname is David, Lastname is Johnson. The unique id will be generate DJ01. How i can archieve this? Any sample?