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.