getting warning on xmlelement while writing xml file in php

I have following code with which i am writing the xml files in php,But when i run this code,I get following error

SimpleXMLElement::asXML(): string is not in UTF-8

Code for generating xml is as follows

<?php

    //Create Database connection
  $mysqli = new mysqli('localhost', 'user', 'pass', 'dbnam');

   if(mysqli_connect_errno()) {
      echo "Connection Failed: " . mysqli_connect_errno();
      }
 $query='select Siteurl from tablename order by colname2 desc ';

    $result = mysqli_query($mysqli,$query);  

    //Create SimpleXMLElement object
    $xml = new SimpleXMLElement('<urls/>');

    //Add each column value a node of the XML object
    while($row = mysqli_fetch_array($result)) {
        $mydata = $xml->addChild('url');
        $mydata->loc=$row['Siteurl'];
    }

  mysqli_close($mysqli);
    //Create the XML file
    $fp = fopen("sitemaps/sitemap2.xml","wb");

    //Write the XML nodes
    fwrite($fp,$xml->asXML());

    //Close the database connection
    fclose($fp);

?>

How can i correct the error guys.Please help me