如何在PHP中做朋友推荐和奖励系统？

as title said, how to do or what are the practices to do a system, that if the user refer to a friends (up to 10), he will get rewards per person in php and mysql? Do I need to do in cookies or database? What are the required columns if using database? How to send multiple emails?as title said, how to do or what are the practi