$query = "SELECT * from table";
$result = $con->query($query);
WHILE($row = $result->fetch_assoc()) {
    $data[] = $row;
}

function array_to_xml( $data, &$xml_data ) {
    foreach( $data as $key => $value ) {
            if( is_numeric($key) ){
                    $key = 'item'.$key; //dealing with <0/>..<n/> issues
            }
            if( is_array($value) ) {
                    $subnode = $xml_data->addChild($key);
                    array_to_xml($value, $subnode);
            } else {
                    $xml_data->addChild("$key",htmlspecialchars("$value"));
            }
     }
}

$xml_data = new SimpleXMLElement('<?xml version="1.0"?><data></data>');
array_to_xml($array,$xml_data);
$result = $xml_data->asXML('test.xml');