What this file does: This file provides the registration form where new test users enter their registration details (Username, Phone Number, Password) and stores them in the database.
Why it is needed: To test payment flows, we need users. Each user has their own balance and deposit history, which lets us simulate real fintech user flows.
Security Note: It uses password_hash() with bcrypt. Never save raw user passwords to a database. It also uses parameterized SQL statements via PDO to secure database queries against SQL Injection attacks.