How many active sessions on the website?

I have a simple page that I can pull up that will show me all of the variables stored in the online session. It’s good for debugging and such. The thought just came to me that there might be a way to see how many active sessions are on my website at a given moment. Is there some easy PHP code I could implement that would show me?

Count the session cookies present in a given time frame (say 15 minutes). That should cover it.