ช่วยดูโค้ดการทำไฟล์ xml ให้ทีค่ะ ทำไมค่ามันไม่ยอมออก

<?php
header("Content-type: text/xml");
require("db.php");

function parseToXML($htmlStr) 
{ 
$xmlStr=str_replace('<','&lt;',$htmlStr); 
$xmlStr=str_replace('>','&gt;',$xmlStr); 
$xmlStr=str_replace('"','&quot;',$xmlStr); 
$xmlStr=str_replace("'",'&#39;',$xmlStr); 
$xmlStr=str_replace("&",'&amp;',$xmlStr); 
return $xmlStr; 
} 

// Get parameters from URL
$center_lat = $_GET["latitude"];
$center_lng = $_GET["longitude"];
$radius = $_GET["radius"];

// 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
$query = sprintf("SELECT  latitude, longitude, ( 3959 * acos( cos( radians('%s') ) * cos( radians( latitude ) ) * cos( radians( longitude ) - radians('%s') ) + sin( radians('%s') ) * sin( radians( latitude ) ) ) ) AS distance FROM place HAVING distance < '%s' ORDER BY distance LIMIT 0 , 20",
  mysql_real_escape_string($center_lat),
  mysql_real_escape_string($center_lng),
  mysql_real_escape_string($center_lat),
  mysql_real_escape_string($radius));

$result = mysql_query($query);
if (!$result) {
  die('Invalid query: ' . mysql_error());
}

// Start XML file, echo parent node
echo "<markers>\n";
// Iterate through the rows, printing XML nodes for each
while ($row = @mysql_fetch_assoc($result)){
  // ADD TO XML DOCUMENT NODE
  echo '<marker ';
  echo 'name_place="' . parseToXML($row['name_place']) . '" ';
  echo 'latitude="' . $row['latitude'] . '" ';
  echo 'longitude="' . $row['longitude'] . '" ';
  echo 'distance="' . $row['distance'] . '" ';
  echo "/>\n";
}

// End XML file
echo "</markers>\n";

?>