What's not mentioned here is that DOMNode::appendChild() can also be used to move an existing node to another part of the DOMDocument, e.g.

<?php
$doc = new DOMDocument();
$doc->loadXML("<foobar><bar/><foo/></foobar>");
$bar = $doc->documentElement->firstChild;
$foo = $doc->documentElement->lastChild;
$foo->appendChild($bar);
print $doc->saveXML();
?>

This produces:

<?xml version="1.0"?>
<foobar><foo><bar/></foo></foobar>

Note that the nodes "<foo/>" and "<bar/>" were siblings, i.e. the first and last child of "<foobar>" but using appendChild() we were able to move "<bar/>" so that it is a child of "<foo/>".

This saves you the trouble of doing a DOMNode::removeChild($bar) to remove "<bar/>" before appending it as a child of "<foo/>". 

Kris Dover

If you want to move the children of one node to another, you cannot simply iterate on $element->childNodes - you have to make an array first:

<?php
$children = [];
foreach ($elemWithChildren->childNodes as $child) {
    $children[] = $child;
}
foreach ($children as $child) {
    $targetElement->appendChild($child);
}
?>

Aware dealing with DOMNodeList and appendChild() on the same Node.

If you want to replace only the children not the DOMElement itself you probably foreach childNodes-property or get the DOMElements with a for-loop and item()-method of the DOMNodeList.

You will fail if you not clone the received single DOMElement. Actually the count of the DOMNodelist will be decreased on appendChild count but appendChild seems to refer to the old Nodelist and nothing visible will happen. Cloning will help.

If you want to create nested DOM elements:

<?php
    $doc = new DOMDocument();
    
    $foo = $doc->createElement("foo");
    $doc->appendChild($foo);

    $bar = $doc->createElement("bar");
    $foo->appendChild($bar);

    $bazz = $doc->createElement("bazz");
    $foo->appendChild($bazz);

    echo $doc->saveXML();
?>

Is equivalent to:

<foo>
  <bar></bar>
  <bazz></bazz>
</foo>