<?php
require_once('XMLDocument.php');

$ns = array(
            'DAV:' => '',
            'urn:ietf:params:xml:ns:caldav' => 'C',
            'http://apple.com/ns/ical/' => 'ical');

$xml = new XMLDocument($ns);

$set = $xml->NewXMLElement('set');
$prop = $set->NewElement('prop');
$xml->NSElement($prop, 'displayname', 'Test calendar');
$xml->NSElement($prop, 'http://apple.com/ns/ical/:calendar-color', '#000000ff');

echo $xml->Render('urn:ietf:params:xml:ns:caldav:mkcalendar', $set);
?>

Result:

<?xml version="1.0" encoding="utf-8" ?>
<mkcalendar xmlns="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav" xmlns:ical="http://apple.com/ns/ical/">
 <set>
  <prop>
   <displayname>Calendario de prueba</displayname>
   <ical:calendar-color>#000000ff</ical:calendar-color>
  </prop>
 </set>
</mkcalendar>