<?php
$xml = new XMLWriter;
$xml->openMemory();
$xml->startDocument('1.0', 'UTF-8');

$xml->startElement("wololo");
$xml->writeAttribute("ver", "1.0");
$xml->endElement();

$xml->endDocument();
echo $xml->outputMemory(TRUE);

<?xml version="1.0" encoding="UTF-8"?>
<wololo ver="1.0"/>