[david_kw]

Ok, I'm an idiot. I just noticed a problem. You weren't calling http.send(null); in your showComments() function.

function showComments()
{
  var http = createRequestObject(); 
  var url = 'callcomment.php';
  http.open("GET", url, false);
  http.send(null);
  alert("status = '" + http.status + "'");
  alert("response = '" + http.responseText + "'");
  var response = http.responseText;
  document.getElementById('existing_comments').innerHTML = response;
}

david_kw