An excerpt from DOM Level 2 Node::insertBefore:
"If the newChild is already in the tree, it is first removed."
 
 
In php 4.2.1 newChild is not removed from tree, it is only copied to the new location.

Here is some sample code,

$xml = <<<EOD
<root>
	<Test>
		data to merge in parent
		bla bla
		<node>asdasd</node>
	</Test>
	<Treta>
		contents
 	</Treta>
</root>
EOD;
 

/* Move all children nodes to parent node of $fromNode. */ function MoveChildrenToParent($fromNode) {
	$parentNode = $fromNode->parent_node();
	foreach ($fromNode->child_nodes() as $childNode) {
		//$fromNode->remove_child($childNode); // <-- uncomment this
		$parentNode->insert_before($childNode, $fromNode);
	}
}

$dom = domxml_open_mem($xml);
$root = $dom->document_element();
$children = $root->child_nodes();
$testNode = $children[1];
MoveChildrenToParent($testNode);
 

As you can see the children of Test node were not moved to root, they were copied. (uncomment the remove_child call from MoveChildrenToParent to see the right behavior)