How to create sub.domain for user opon sign up

[rickynewyork]

I am using php/mysql on linux servers. I want to create a user sub domain for each user upon sign up. For instance, john doe --> johndoe.example.comThe issue is, we are going to allow customizations such as adding custom features for clients wanting customizations. So, is it better we automate the process or manually create sub domains for clients requiring customizations and let other users just use basic login?And how can i automate the process if need be?