title: xhttp is not defined when using ajax calls into an iframe
body: <p>Good day all, I'm developing a php page in which there is an iframe, that opens another php page with a checkbox in it, this second page, when the user click on the checkbox, has to make an ajax call to confirm the "click".</p>
  
  <p>so there is pageA.php, with a iframe in it that points to pageB.php, in this one, there is only a form with a checkbox and a javascript (vanilla javascript), that sould call a third page on click.</p>
  
  <p>this is the javascript I'm using to send the "click":</p>
  
  <pre><code>document.getElementById("checkboxMe").onclick = function() {
      xhttp.open("POST", "pageC.php", true);
      xhttp.send("foo=bar");  
      };
  </code></pre>
  
  <p>when clicking on the checkbox, this is what I see on the console:</p>
  
  <pre><code>Uncaught ReferenceError: xhttp is not defined
  </code></pre>
  
  <p>it never happen something like this, infact I can't find this error easily on google, does anyone has some clues? 
  maybe is the fact I'm into an iframe?
  how could I solve this issue?</p>
  
  <p>thanks in advance ppl.</p>