title: php session management
body: <p>I am developing a system in php which has different types of users like admin, A, B, C.</p>
  
  <p>I need to allow log in just one user from the type at a time. Means 
  1) if there is already an admin user is logged in, no other admin user can log in. 
  2) And the logged in credentials cannot be used on other pc at the same time (just like yahoo messenger login system).</p>
  
  <p>I have tried to store session with login logout time in database table when someone login and logout. 
  But it creates problem when someone close the browser without logging out, the logout entry time misses.</p>