Is there a way to create a node that has mixed XML content in it with the PHP DOM?
If I understood you correctly you want something similar to innerHTML in JavaScript. There is a solution to that:
$xmlString = 'some <b>mixed</b> content';
$dom = new DOMDocument;
$fragment = $dom->createDocumentFragment();
$fragment->appendXML($xmlString);
$dom->appendChild($fragment);
// done
To sumarize. What you need is:
Although you didn't asked about it I'll tell you how to get the string representation of a DOM node as opposed to the whole DOM document:
// for a DOMDocument you have
$dom->save($file);
$string = $dom->saveXML();
$dom->saveHTML();
$string = $dom->saveHTMLFile($file);
// For a DOMElement you have
$node = $dom->getElementById('some-id');
$string = $node->C14N();
$node->C14NFile($file);
Those two methods are currently not documented.