If you will write a HTML document :
// $xml = new XMLWriter(); you know it and openURI()
<?php
// for HTML 5
$xml->startDTD('html');
// for XHTML 1.0
$xml->startDTD('html', '-//W3C//DTD XHTML 1.0 Strict//EN','http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd'); // standards compliant
$xml->endDTD();
?>
it will be :
<!DOCTYPE html>
or <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN">
don't need to startDocument() .