HTTP Session Management while using Nginx as in “Round Robin” mode Load-balancer?

I'm trying to load-balance "2 Web Servers (running Apache/PHP)" by putting nginx in front of them. But **I need to use Round Robin algorithm** but when I do this, **I can't manage to have the stable SESSIONS**. (I understand; if I use Round Robin, the session information will be lost once I hit to the another server on next load) Is there a proper way to achieve this? Any kind advice for the industrial standards on this please? *FYI, I've already put these 2 Web Servers into GlusterFS as in Cluster. So I have a common storage (if you are going to suggest something based on this)*