Description:
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DOMDocumentFragment->appendXML() creates elements in default namespace (if they don't have namespace set explictly, ofcourse).

However elements in XML fragments may just rely on namespace being set on parent element and don't  define their own namespace, so it would be very useful it if was possible to specify implied namespace, eg.

$fragment->appendXML('<p>test</p>',"http://www.w3.org/1999/xhtml");

should be equivalent to

$fragment->appendXML('<p xmlns="http://www.w3.org/1999/xhtml">test</p>');