Simple PHP and XML data show, $xml->flush();

Dear Experts,
I use below code to select the merchants to display them on the map.

If I select the 15-20 data the result shows up, but if I select more than 20 data the below xml code is not working.

I don't understand why it is not working with more than 20 data?
What do you suggest I should do?

what I need is to show it with XML.
thank you


$mysqli->query("set names utf8mb4");

$sql = "I select here the merchants";

$res = $mysqli->query($sql);

$xml = new XMLWriter();

$xml->openURI("php://output");
$xml->startDocument();
$xml->setIndent(true);

$xml->startElement('markers');

while ($row = mysqli_fetch_assoc($res)) {
  $xml->startElement("marker");

 $xml->writeAttribute('id', $row['uid']);
  $xml->writeAttribute('name', $row['isim']);
 $xml->writeAttribute('address', $row['adres']);
  $xml->writeAttribute('lat', $row['lat']);
  $xml->writeAttribute('lng', $row['longt']);
  $xml->writeAttribute('type', $row['bayituru']);
  //$xml->writeRaw($row['lng']);

  $xml->endElement();
}

$xml->endElement();

header('Content-type: text/xml');
$xml->flush();