function fetch2xml($sql) { 
    $q = mysql_query($sql); 
    $doc = new DOMDocument('1.0', 'UTF-8'); 
    $root = $doc->appendChild($doc->createElement('result')); 
    while ($qrow = mysql_fetch_assoc($q)) { 
        $r = $root->appendChild($doc->createElement('r')); 
        foreach($qrow as $name => $value) { 
            // Не включаем поля со значением NULL 
            if ($value !== NULL) { 
                $f = $r->appendChild($doc->createElement($name)); 
                $f->appendChild($doc->createTextNode($value)); 
            } 
        } 
    } 
    return $doc; 
} 

mysql_connect(...); 
mysql_select_db(...); 
$doc = fetch2xml(...); 
header("Content-Type: text/xml; charset=UTF-8"); 
echo $doc->saveXML();