php source code for viral/referral user/community process

hi. looking over the net/github fir php base algo/code to implement a referral/rewards process. user sends invite email to others. the parentRefID is used to track/manage relationships between parent/chil user. Process continues with child sending invites to others, etc. the rewards process is based on the num of email sent, for diff levels of rewards. before I jump into doing this, wanted to see if a recent chunk of working code existed that I could use as a basis. thanks ps. there are plenty of half baked sons, but I haven't found any solid ones yet.