The problem is DOM does not know that it should consider the XHTML DTD unless you validated the document against it. Unless you do that, DOM doesnt know any entities defined in the DTD, nor any other rules in it. Fortunately, we sorted out how to do the validation in that other question, so armed with that knowledge you can do
$document->validate(); // anywhere before importing the other DOM
And then import with
$fragment = $document->createDocumentFragment();
$fragment->appendXML('<h1>Hello</h1><p>Hello World</p>');
$document->getElementsByTagName('body')->item(0)->appendChild($fragment);
$document->formatOutput = TRUE;
echo $document->saveXml();
outputs:
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>My bweb page</title>
</head>
<body>
<h1>Hello</h1>
<p>Hello World</p>
</body>
</html>
The other way to import XML into another DOM is to use
$one = new DOMDocument;
$two = new DOMDocument;
$one->loadXml('<root><foo>one</foo></root>');
$two->loadXml('<root><bar><sub>two</sub></bar></root>');
$bar = $two->documentElement->firstChild; // we want to import the bar tree
$one->documentElement->appendChild($one->importNode($bar, TRUE));
echo $one->saveXml();
outputs:
<?xml version="1.0"?>
<root><foo>one</foo><bar><sub>two</sub></bar></root>
However, this cannot work with
<h1>Hello</h1><p>Hello World</p>
because when you load a document into DOM, DOM will overwrite everything you told it before about the document. Thus, when using load, libxml (and thus SimpleXml, DOM and XMLReader) does (do) not know you mean XHTML. And it does not know any entities defined in it and will fuzz about them instead. But even if the string would not contain the entity, it is not valid XML, because it lacks a root node. That’s why you use the fragment.