// holds an instance of XMLHttpRequest
var xmlHttp = createXmlHttpRequestObject();

// creates an XMLHttpRequest instance
function createXmlHttpRequestObject() 
{
  // will store the reference to the XMLHttpRequest object
  var xmlHttp;
  // this should work for all browsers except IE6 and older
  try
  {
    // try to create XMLHttpRequest object
    xmlHttp = new XMLHttpRequest();
  }
  catch(e)
  {
    // assume IE6 or older
    var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                    "MSXML2.XMLHTTP.5.0",
                                    "MSXML2.XMLHTTP.4.0",
                                    "MSXML2.XMLHTTP.3.0",
                                    "MSXML2.XMLHTTP",
                                    "Microsoft.XMLHTTP");
    // try every prog id until one works
    for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++) 
    {
      try 
      { 
        // try to create XMLHttpRequest object
        xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
      } 
      catch (e) {}
    }
  }
  // return the created object or display an error message
  if (!xmlHttp)
    alert("Error creating the XMLHttpRequest object.");
  else 
    return xmlHttp;
}

function display_price(attstr){
  // only continue if xmlHttp isn't void
  if (xmlHttp)
  {    
        xmlHttp.open("GET", 'ajaxPrice.php'+ "?" + 'attribute=' + attstr, true);
        xmlHttp.onreadystatechange = function(){
            displayResult();
            }  
        xmlHttp.send(null);
      }
 }

 function displayResult(){
 if (xmlHttp.readyState == 4) {
            var response = xmlHttp.responseText;
            document.getElementById("attr_price").innerHTML = response;
            document.getElementById("attr_price_inc").innerHTML = calcVAT(response);
 }
 } 
 
 function calcVAT(price){
     var taxRate = getTaxRate();
     if (taxRate > 0) {
        grossValue = price * ((taxRate / 100) + 1);
     }
     return doRound(grossValue, 2);
 }
 
 function doRound(x, places) {
  return Math.round(x * Math.pow(10, places)) / Math.pow(10, places);
}

function getTaxRate() {
  if ( tax_rate > 0) {
    return tax_rate;
  } else {
    return 0;
  }
}

function doRound(x, places) {
  return Math.round(x * Math.pow(10, places)) / Math.pow(10, places);
}