String parsing error while writing an xml file in php - php

Question

I am writing an sitemap file with php code.I am able to write the file correctly according to requirement.But i want to add namespace tag to starting node of xml file,I tried it doing by the code.But got following error
Uncaught exception Exception with message 'String could not be parsed as XML'
following is my xml structure
<urlset>
<url>
<loc>url1</loc>
</url>
..
..
</urlset>
There are number of url fragments under the parent node as urlset.I want to add namespace tag for sitemaps to urlset node.Thats the only thing i am not able to.Xml file is quite bigger,i am not able to do it manually..
following is my xml formation code
<?php
//Create Database connection
$mysqli = new mysqli('localhost', 'user', 'pass', 'dbname');
if(mysqli_connect_errno()) {
echo "Connection Failed: " . mysqli_connect_errno();
}
/* if (!$mysqli->set_charset("utf8")) {
printf("Error loading character set utf8: %s\n", $mysqli->error);
} else {
printf("Current character set: %s\n", $mysqli->character_set_name());
}
*/
$xml = new SimpleXMLElement('<xml/>');
for ($i = 0; $i < 2; $i++) {
// used to be: $query='select tablecolname1 from tablename limit 10000';
$start = $i * 50000;
$query = "select Siteurl from tablename limit $start, 50000";
$result = mysqli_query($mysqli,$query);
//Create SimpleXMLElement object
$xml = new SimpleXMLElement('<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">');
while($row = mysqli_fetch_array($result)) {
$mydata = $xml->addChild('url');
$mydata->loc=$row['Siteurl'];
//htmlentities(strip_tags($mydata->loc=$row['Siteurl']), ENT_COMPAT,'utf-8');
}
// used to be: $fp = fopen("folder/file2.xml","wb");
$fp = fopen("site/sitemap$i.xml","wb");
fwrite($fp,utf8_encode($xml->asXML()));
fclose($fp);
}
$xml = new SimpleXMLElement('<urlset/>');
?>
Please guide me on how to add namespace tag to urlset tag..

Answer 1

SimpleXMLElement expects as parameter "a well-formed XML string" according to the documentation.
'<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">' is not such a string.
You can build a XML from PHP in (at least) two (simple) ways:
either only by echoing the content you want in your XML file (tags included):
header('Content-Type:text/xml');
echo '<?xml version="1.0" encoding="UTF-8"?>';
echo '<urlset xmlns="...">';
...
echo '</urlset>';
or by using the SimpleXML object and building the DOM tree (see example #10 in documentation):
header('Content-Type:text/xml');
$xml=new SImpleXMLElement('<xml/>');
$urlset=$xml->addChild('urlset');
$urlset->addAttribute('xmlns','...');
...
echo $xml->asXML();