SimpleXML: append one tree to another

I have two XML trees and would like to add one tree as a leaf to the other one.

Apparently:

$tree2->addChild('leaf', $tree1);

doesn't work, as it copies only the first root node.

Ok, so then I thought I would traverse the whole first tree, adding every element one by one to the second one.

But consider XML like this:

<root>
  aaa
  <bbb/>
  ccc
</root>

How do I access "ccc"? tree1->children() returns just "bbb"... .