simpleXML - can't add children - why? - stuck

Hello all, I'm struggling here, but no joy so far. Having this XML file that I'm loading using simplexml_load_file(): http://pastebin.com/JiW3LxWM I need to add some <domain:create> children nodes after line 8. What do I need to add? This: http://pastebin.com/mT27g3ZN In order to do so, I have used the following php snipped: http://pastebin.com/a1gG8tXx I'm getting this warning and, after it, a fatal error: Warning: SimpleXMLElement::addChild() [simplexmlelement.addchild]: Cannot add child. Parent is not a permanent member of the XML tree. (line number 1 of last paste bin) Fatal error: Call to a member function addChild() on a non-object (line number 2 of last paste bin) Note that: The constant used is correct. The php version is higher then 5.1. On the same command, later on this code, I was able to create child nodes with no issues. I'm far (veery far) from being a guru, so please, what can I do to debug this, and find out what's going on? Thanks a lot, Márcio