title: Send notification from server to client on server event
body: <p>How can I send a message from server to client with PHP, avoiding extraneous Ajax calls.</p>
  
  <p>Here is the idea:</p>
  
  <ol>
  <li><p>User: Alice makes a change which gets sent to the server.</p></li>
  <li><p>The Server then checks to see which users are not up to date, and if not calls some code to send the information pertaining to the change to Bob (who is not up to date in this case).</p></li>
  </ol>
  
  <p><strong>How do I send Bob the message?</strong></p>