PHP SimpleXML: insert node at certain position

say I have XML:

<root>
  <nodeA />
  <nodeA />
  <nodeA />
  <nodeC />
  <nodeC />
  <nodeC />
</root>

How do I insert "nodeB" between As and Cs? In PHP, preferably via SimpleXML? Like:

<root>
  <nodeA />
  <nodeA />
  <nodeA />
  <nodeB />
  <nodeC />
  <nodeC />
  <nodeC />
</root>