ok I found my one way .. I did it with FILES via php: this is the code if anyone needs help to that:

<?php
include "connection.php";
$myFile = "rss.xml";
$fh = fopen($myFile, 'w') or die("can't open file");

$rss_txt .= '<?xml version="1.0" encoding="utf-8"?>';
$rss_txt .= "<rss version='2.0'>";
$rss_txt .= '<channel>';
    $query = mysql_query("SELECT * FROM rssfeeds");
    while($values_query = mysql_fetch_assoc($query))
    {
        $rss_txt .= '<item>';

        $rss_txt .= '<title>' .$values_query['title']. '</title>';
        $rss_txt .= '<link>' .$values_query['link']. '</link>';
        $rss_txt .= '<description>' .$values_query['summary']. '</description>';

        $rss_txt .= '</item>';
    }
$rss_txt .= '</channel>';
$rss_txt .= '</rss>';

fwrite(rss_txt);
fclose($fh);
?>

An example with XMLWriter.

mysql_connect('server', 'user', 'pass');
mysql_select_db('database');

$sql = "SELECT udid, country FROM table ORDER BY udid";
$res = mysql_query(xml = new XMLWriter();

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

$xml->startElement('countries');

while ($row = mysql_fetch_assoc($res)) {
  $xml->startElement("country");

  $xml->writeAttribute('udid', $row['udid']);
  $xml->writeRaw($row['country']);

  $xml->endElement();
}

$xml->endElement();

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

Output:

<?xml version="1.0"?>
<countries>
 <country udid="1">Country 1</country>
 <country udid="2">Country 2</country>
 ...
 <country udid="n">Country n</country>
</countries>

I'd use SimpleXMLElement.

<?php

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

for ($i = 1; $i <= 8; ++$i) {
    $track = $xml->addChild('track');
    $track->addChild('path', "song$i.mp3");
    $track->addChild('title', "Track $i - Track Title");
}

Header('Content-type: text/xml');
print($xml->asXML());

?>

$xml->asXML() can also take a filename as a parameter to save to that file

It should work to just put the creation of the document inside the loop. By adding the loops number $i to the filepath, you prevent overwriting the file on each iteration.

function createXMLfile($stackarray){
    for($i=0; $i<count($stackarray); $i++){
        $filePath = "/root/stack" . $i . ".xml";
        $dom = new DOMDocument('1.0', 'utf-8');
        $root = $dom->createElement('REQUEST');
        $stackName    =  $stackarray[$i]['stack_name']; 
        $stackSERIAL      =  $stackarray[$i]['stack_serialnumber']; 
        $content = $dom->createElement('CONTENT');
        $device = $dom->createElement('DEVICE');
        $info = $dom->createElement('INFO');
        $name   = $dom->createElement('NAME', $stackName); 
        $info->appendChild($name);  
        $serial     = $dom->createElement('SERIAL', $stackSERIAL); 
        $info->appendChild($serial);
        $type     = $dom->createElement('TYPE', "NETWORKING"); 
        $info->appendChild($type);
        $device->appendChild($info);
        $root->appendChild($content);
        $module     = $dom->createElement('MODULEVERSION', "3.1");
        $content->appendChild($module);
        $process     = $dom->createElement('PROCESSNUMBER', "1");
        $content->appendChild($process);
        $content->appendChild($device);        
        $deviceid = $dom->createElement('DEVICEID', "foo");
        $root->appendChild($deviceid);
        $query     = $dom->createElement('QUERY', "SNMPQUERY");
        $root->appendChild($query);
        $dom->appendChild($root); 
        $dom->save($filePath);
    }
}

I would suggest not adding .xml in .htaccess to be processed using PHP compiler. Rather use mod_rewrite to redirect requests to .xml files to php script.

For example:

RewriteEngine On
RewriteRule ^(.*)\.xml$ /rss_feed.php [L,QSA]

I struggle a lot to find out this solution in mysqli format but nowhere i found the solution. Below is the solution i figured. Run this demo and map it your requirement, surely it will help.

<?php
//Create file name to save
$filename = "export_xml_".date("Y-m-d_H-i",time()).".xml";

$mysql = new Mysqli('server', 'user', 'pass', 'database');
if ($mysql->connect_errno) {
    throw new Exception(sprintf("Mysqli: (%d): %s", $mysql->connect_errno, $mysql->connect_error));
}

//Extract data to export to XML
$sqlQuery = 'SELECT * FROM t1';
if (!$result = $mysql->query($sqlQuery)) {
    throw new Exception(sprintf('Mysqli: (%d): %s', $mysql->errno, $mysql->error));
}

//Create new document 
$dom = new DOMDocument;
$dom->preserveWhiteSpace = FALSE;

//add table in document 
$table = $dom->appendChild($dom->createElement('table'));

//add row in document 
foreach($result as $row) {
    $data = $dom->createElement('row');
    $table->appendChild($data);

    //add column in document 
    foreach($row as $name => $value) {

        $col = $dom->createElement('column', $value);
        $data->appendChild($col);
        $colattribute = $dom->createAttribute('name');
        // Value for the created attribute
        $colattribute->value = $name;
        $col->appendChild($colattribute);           
    }
}

/*
** insert more nodes
*/

$dom->formatOutput = true; // set the formatOutput attribute of domDocument to true 
// save XML as string or file 
$test1 = $dom->saveXML(); // put string in test1
$dom->save($filename); // save as file
$dom->save('xml/'.$filename);   
?>