How to develop a friends finder for social networking in PHP

I want to develop a friends finder that开发者_高级运维 can get the friends list from other mail services like gmail or something. I can use php and mysql. But, I have no ideas to start. It may be to develop with php?. How can I try? Let me know any ideas.

---

Try http://openinviter.com/

Free, open-source, easy to setup and well-maintained.