title: Change session ID and keep data?
body: <p>When a user logs in on our website, I want to change the session ID but keep whatever data is in the session. I want to do this for two reasons:</p>
  
  <ol>
  <li>To prevent a user account to be used at multiple places simultaneously (because if two people are using the same account, the actions of one will undermine the actions of the other).</li>
  <li>To let the user continue what he/she was doing on another computer (e.g moving from home computer to work).</li>
  </ol>
  
  <p>These might seem contradictory, but really aren't if you think it through. </p>
  
  <p>The problem is as follows; to get to the data that is currently in the session, I have to call <code>session_start()</code>. This means I cannot call <code>session_id()</code> afterwards to set a new session ID.  Any ideas how to transfer the session data and change the session ID.</p>
  
  <p><em>Update</em>: I need to be able to choose the session ID myself. <code>session_regenerate_id()</code> therefore won't work.</p>