How to capture a whole page through Ajax

I have a list box in a site with which I capture a selected value with the onChange event using the capture_value() function (code listed below).
This function passes 2 arguments, i.e., 'str' which is the selected list box value and 'passed_url' which is a passed url for running a php script (which contains some url query parameters, e.g. 'somescript.php?var1=value1&var2=value2&var3=value3').

The capture_value() function actually sends a Ajax request with the GET method and a URL which is formed by appending the captured list box value to the 'passed_url' url value (i.e. the 'passed_url' is changed to
'somescript.php?var1=value1&var2=value2&var3=value3&str=value4'.
Now, what I want to do is to capture the whole generated page from running the final 'passed_url' in the same window (as if I entered 'somescript.php?var1=value1&var2=value2&var3=value3&str=value4' in the location
field of the browser and pressed <Enter>).

In other words, I want to output in the same window the generated page from running the 'somescript.php?var1=value1&var2=value2&var3=value3&str=value4' url address.

Below is the code listing of the Javascript file I use.
----------------------------------------------------------------------------------

var xmlhttp;

function capture_value(passed_url,str)
{
xmlhttp=GetXmlHttpObject();
if (xmlhttp==null)
  {
  alert ("Browser does not support HTTP Request ");
  return;
  }
//var url="http://www.test.local/ach_handcrfts/product.php";
passed_url=passed_url+"&drawing_style="+str;
passed_url=passed_url+"&sid="+Math.random();
//alert ("url: " + url);
xmlhttp.onreadystatechange=stateChanged;
xmlhttp.open("GET",passed_url,true);
xmlhttp.send(null);

}

function stateChanged()
{
if (xmlhttp.readyState == 4) {
        if (xmlhttp.status == 200) {
           window.open(xmlhttp.responseXML,"_self");
       
        } else {
            alert("Response Error:n" + xmlhttp.statusText);
        }
    }

}

function GetXmlHttpObject()
{
if (window.XMLHttpRequest)
  {
  // code for IE7+, Firefox, Chrome, Opera, Safari  
  return new XMLHttpRequest();
  }
if (window.ActiveXObject)
  {
  // code for IE6, IE5
  return new ActiveXObject("Microsoft.XMLHTTP");
  }
return null;
}

Code (markup):

Now, when I select a value in the list box, the desired page is not generated (nothing happens). When I check
with Firebug, I can see that I have no errors, and
I get the expected GET response in the Firebug console, and when I click on it's console link with the right
button and then click on "Open in New Tab", a new browser window opens with the expected page, but this page is not generated by selecting a value in the list box, i.e. somewhere in the end of the code I need to add some code to output the whole page in the same window.

I am sure it's something simple missing or needing modification.
Could you please somebody help?

 

Though it might not be the problem, i usually write in the order,

rather than,

Try whether it works or not.
Hope this helps.