无法使用ajax解析json

Question

I am providing a sample code here where I am sending data to php script in response I am able to get values in data from xmlhttp responseText But the issue is I am not able to parse it using JSON parse

Here is the code :

I am able to alert data

var xmlhttp;
if (window.XMLHttpRequest)
{
    // code for IE7+, Firefox, Chrome, Opera, Safari
    xmlhttp=new XMLHttpRequest();
}                               
else
{
    // code for IE6, IE5
    xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.onreadystatechange=function()
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {   //alert("hiiiiiiiii");
            var data = xmlhttp.responseText;  
            alert(data);
            //alert(data123.taxi_sr_no_array);
            obj=JSON.parse(data);
            //alert("object is :" + obj);
            //alert(obj.taxi_no_of_rows+' 
 '+obj.taxi_sr_no_array+' 
 '+obj.taxi_imei_number_array+'
 '+obj.taxi_latitude_array+'
'+obj.taxi_longitude_array+' 
 '+obj.taxi_hemisphere_array);
            alert(obj.taxi_no_of_rows);
         }
     }
xmlhttp.open("GET","<?php echo site_url()."/welcome/taxi_ajax_data";?>",true);
xmlhttp.send();