title: JSON Response headers
body: <p>The following headers seem to be recommended using when outputing JSON:</p>
  
  <pre><code>header('Cache-Control: no-cache, must-revalidate');
  header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');
  header('Content-type: application/json');
  </code></pre>
  
  <p>What is the purpose of the Expires header when Cache-control is set to no-cache, must-revalidate?</p>