无法更新MySQL数据库中的单元格？

I have a google maps web application for a game where the user will click a google map marker, make a selection in the window that pops up and click submit. It uses AJAX to update the database with information selected by the user. The database is pre-populated with names of the markers and GPS coordinates, which are loaded. The markers are also placed accordingly upon load via XML.

I'm having trouble updating one row in my DB called quest with the user selected information when it's submitted. Currently, a user can select a marker and submit a quest, but it won't update the DB at all. I'm unsure on the correct WHERE statement to use. Here's my current SQL statement, I'm trying to update a row called quest.

mysqli_query($con, "UPDATE markers SET quest= '$questName'  WHERE markerName = '$markerName'");

Here's my downloadURL function inside of myMap function

downloadUrl("call2.php", function(data) {
    var xml = data.responseXML;
    var markers = xml.documentElement.getElementsByTagName("marker");
    for (var i = 0; i < markers.length; i++) {
      var name = markers[i].getAttribute("markerName");
    //  var address = markers[i].getAttribute("address");
  var type = markers[i].getAttribute("type");
  var point = new google.maps.LatLng(
      parseFloat(markers[i].getAttribute("lat")),
      parseFloat(markers[i].getAttribute("longg")));
  var icon = customLabel[type] || {};
  var marker = new google.maps.Marker({
    map: map,
    position: point,
    icon: icon.icon,
    shadow: icon.shadow
  });
  bindInfoWindow(marker, map, infoWindow, html);
}
});

This is what happens when the submit button is pressed.

if (document.getElementById("questType").value == 
"quest1") { //if quest1 is selected upon submission
alert("quest1");
var markerName;
var questName = "Quest 1";

xmlhttp = new XMLHttpRequest(); //Creating the 
XMLHttpRequest object           
xmlhttp.open("GET", "ajax.php?questName=" + questName 
+ "&markerName=" + markerName, true); //Specifying the type of request to the server                    
xmlhttp.send(); //Sending the request to the server

Here is my relevant ajax.php.

<?php 
include("connect.php");
require("call.php");

if (mysqli_connect_errno()) {
    echo "Failed to connect to MySQL: " . 
mysqli_connect_error();     
} else  {   

} 

$markerName = mysqli_real_escape_string($con, 
$_GET['markerName']);
$questName = mysqli_real_escape_string($con,$_GET['questName']);
$lat = mysqli_real_escape_string($con, $_GET['lat']);
$longg = mysqli_real_escape_string($con, $_GET['longg']);
mysqli_query($con, "UPDATE markers SET quest= 
'$questName'  WHERE markerName = '$markerName'");
mysqli_close($con);

Here is my relevant call file as well.

$dom     = new DOMDocument("1.0");
$node    = $dom->createElement("markers");
$parnode = $dom->appendChild($node);
include('connect.php');
$query  = "SELECT * FROM markers WHERE 1"; // Select all the rows in the markers table
$result = mysqli_query($con, $query);
if (!$result) {
die('Invalid query: ' . mysqli_error($con));
}

// Iterate through the rows, adding XML nodes for each
while ($row = mysqli_fetch_assoc($result)) {
global $dom, $node, $parnode;

    $node    = $dom->createElement("marker");
    $newnode = $parnode->appendChild($node);
    $newnode->setAttribute("markerName", 
    $row['markerName']);
    $newnode->setAttribute("quest", $row['quest']);
    $newnode->setAttribute("lat", $row['lat']);
    $newnode->setAttribute("longg", $row['longg']);
}

header("Content-type: text/xml");
echo $dom->saveXML();

I don't get any errors, however when I hover over the ajaxphp in the network tab on chrome it looks like it's getting the questName but markerName remains undefined.