我得到“documentElement of null”错误。我正在努力跟随https://developers.google.com/maps/articles/phpsqlajax_v3 更新：在使用dev工具时，“downloadUrl（）”函数似乎有问题。

我的“phpsqlajax\\u genxml.php”创建\\u元素（“标记”）$parnode=$doc->append\\u child（$node）；

// Opens a connection to a mySQL server
/*$connection=mysql_connect (localhost, $username, $password);
if (!$connection) {
  die(\'Not connected : \' . mysql_error());
}

// Set the active mySQL database
$db_selected = mysql_select_db($database, $connection);
if (!$db_selected) {
  die (\'Can\\\'t use db : \' . mysql_error());
}*/

// Select all the rows in the markers table
global $wpdb ;
//$query = "SELECT * FROM markers WHERE 1";
$query = $wpdb->get_results("SELECT * FROM `markers` WHERE 1;") ;
/*no need of this
$result = mysql_query($query);
if (!$result) {
  die(\'Invalid query: \' . mysql_error());
}*/

header("Content-type: text/xml");

// Iterate through the rows, adding XML nodes for each
 foreach ($query as $q) 
  {
  // ADD TO XML DOCUMENT NODE
  echo \'<marker \';
  echo \'name="\' . parseToXML($q->name) . \'" \';
  echo \'address="\' . parseToXML($q->address) . \'" \';
  echo \'lat="\' . $q->lat . \'" \';
  echo \'lng="\' . $q->lng . \'" \';
  echo \'type="\' . $q->type . \'" \';
  echo \'/>\';
}
    ?>

 <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY"
        type="text/javascript"></script>
<script type="text/javascript">
//<![CDATA[

var customIcons = {
  restaurant: {
    icon: \'http://labs.google.com/ridefinder/images/mm_20_blue.png\'
  },
  bar: {
    icon: \'http://labs.google.com/ridefinder/images/mm_20_red.png\'
  }
};

function load() {
  var map = new google.maps.Map(document.getElementById("map"), {
    center: new google.maps.LatLng(47.6145, -122.3418),
    zoom: 13,
    mapTypeId: \'roadmap\'
  });
  var infoWindow = new google.maps.InfoWindow;

  // Change this depending on the name of your PHP file
  downloadUrl("phpsqlajax_genxml.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("name");
      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("lng")));
      var html = "<b>" + name + "</b> <br/>" + address;
      var icon = customIcons[type] || {};
      var marker = new google.maps.Marker({
        map: map,
        position: point,
        icon: icon.icon
      });
      bindInfoWindow(marker, map, infoWindow, html);
    }
  });
}

function bindInfoWindow(marker, map, infoWindow, html) {
  google.maps.event.addListener(marker, \'click\', function() {
    infoWindow.setContent(html);
    infoWindow.open(map, marker);
  });
}

function downloadUrl(url, callback) {
  var request = window.ActiveXObject ?
      new ActiveXObject(\'Microsoft.XMLHTTP\') :
      new XMLHttpRequest;

  request.onreadystatechange = function() {
    if (request.readyState == 4) {
      request.onreadystatechange = doNothing;
      callback(request, request.status);
    }
  };

  request.open(\'GET\', url, true);
  request.send(null);
}

function doNothing() {}

//]]>