title: security issue: php login/logout script + back button
body: <p>I am using a login script that stores a session value </p>
  
  <pre><code>$_SESSION['logged_in'] = 'yes' 
  </code></pre>
  
  <p>when user enters correct user/password. On logout, this session value is set to  blanks.  However user can hit back buttin a few times and get logged back in.  I have also tried  $_SESSION = array(); and  session_destroy(); but with same results.</p>