how to use json array in javascript [duplicate]

Possible Duplicate:
I have a nested data structure / JSON, how can I access a specific value?

i have a javascript which does a XMLHttp request to a php file, which in turn queries a mysql table and returns a JSON array... but somehow i cannot access individual elements of the array. here is the javascript:

<script type="text/javascript">
function show(){
    if(window.XMLHttpRequest){
        xmlhttp=new XMLHttpRequest();
    }
    xmlhttp.onreadystatechange= function () {
        if(xmlhttp.readyState==4 && xmlhttp.status==200){

            var response= new Array();
            response=xmlhttp.response;
            alert(response);
        }
    }
    xmlhttp.open('GET','test.php', true);
    xmlhttp.send();

}
</script>

and this is the php script:

mysql_select_db('testpolaroid') or die ('Unable to select database!');
$query = 'SELECT * FROM images';
$result = mysql_query($query) or die ('Error in query: $query. ' . mysql_error());
if(mysql_num_rows($result) > 0)
{
    $row = mysql_fetch_assoc($result);
    echo json_encode($json);
}

the response that i get is :

 {"imageid":"11","location":"11.jpg"}