title: php, how to detect is a session ID is dead or alive?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/520237/how-do-i-expire-a-php-session-after-30-minutes">How do I expire a PHP session after 30 minutes?</a>  </p>
  </blockquote>
  
  
  
  <p>How to detect is a session ID is dead or alive?
  Lets suppose somebody logged in - a new session is created and I save the session ID. If he logins from another browser, I can determine that he is logged in twice. But how to detect if he logs out?</p>