PHP Membership Script for SMS site

Hi! I'm going to need an SMS sender site, where people can signup for free with email confirmation, and they can login with SESSION-s. On their account home, they see available credits, and etc. They can also refer others with a unique referral link (userid) It should be a simple script, handling only the signups / logins / referrals, I will do the rest (handling sms, credit uploads, etc). Should be done in mySQL / php. Max budget is $40