$dom = new DomDocument('1.0','iso-8859-15');
$ht_ml = $dom->appendChild($dom->createElement('html'));
$ht_ml->setAttribute('xmlns','http://www.w3.org/1999/xhtml');
$ht_ml->setAttribute('xml:lang','fi');
$ht_ml->setAttribute('lang','fi');
Result:
<?xml version="1.0" encoding="iso-8859-15"?>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="fi" lang="fi">