function refresh_filter_combo(p_combo_id, p_filter_name, p_filter_value) {
  var select = document.getElementById(p_combo_id);
  select.innerHTML = get_filter_options(p_filter_name, p_filter_value);
}

function get_filter_options(p_filter_name, p_filter_value) {
  var result = '';
  try {
    // Moz supports XMLHttpRequest. IE uses ActiveX. 
    // browser detction is bad. object detection works for any browser
    xmlhttp = window.XMLHttpRequest?new XMLHttpRequest():new ActiveXObject('Microsoft.XMLHTTP');
    xmlhttp.open('GET', '../_application/src/provider/product_combo/index.php?filtername=' + p_filter_name + '&filtervalue=' + p_filter_value, false);
    xmlhttp.send(null);
    if ((xmlhttp.readyState == 4) && (xmlhttp.status == 200)) {
      result = unescape(xmlhttp.responseText);
    }
  } catch (e) {
   // browser doesn't support ajax. handle however you want
  }
  return result;
}