title: Counting simultaneous page users
body: <p>As part of a treasure hunt style game I'm using some PHP web pages, I'm only prepared to process a form when I know that 10 people (or more) have that page open (they may  not be looking at it of course!) This may take some time so some people may have gone to the page then given up and closed their browser.
  It's easy to create a list of started Session IDs accessible from any any session but I wonder how to identify and remove session IDs that are no longer in use? </p>
  
  <p>I tried to think out a solution using timestamps and ajax but found I was going round in circles</p>