customer sign up script, then complete offers for cash ( i need a script for this)

I need a script that will allow customers to sign up and create an account. When they have joined it will display offers for them to complete that i have made avalible to them. However i will put the URL of the offer in http://OFFER-URL then what i need this script to do is then add the following to the offer url's &USERID for example when the user logs in it will add &USER-ID to the offer link this enables me to track the customers. I will be giving my customers incentives to complete the offers like FREE CASH for X offer or maybe a free CD Player for completing another offer. If you think you have a script for this then please contact me or if you think that you can develope one then tell me as well. I dont want something that someone is going to be able to HACK! And i also dont want to pay too much for this script, however one thing you can be sure of is repeat work, when it takes off i will want a better script that does more when i have made some money. Thats when you will earn more. Seriouse bids only please.