Before a new sibling is added it is first duplicated. Therefore the new child is a completely new copy which can be modified without changing the node which was passed to this function. If the node passed has children itself, they will be duplicated as well, which makes it quite easy to duplicate large parts of an XML document. The return value is the added sibling. If you plan to do further modifications on the added sibling you must use the returned node.
This function has been added to provide the behaviour of domnode_append_child() as it works till PHP 4.2.
See also domnode_append_before().