$node->parent->parent->parent->first_child->next_sibling? Looks frail to me. What if the order gets different? What if the next version of the XML inserts a new tag somewhere? Most likely there is a better way to write this. Show us the XML.

Another thing. There is no point in writing "$variable" where $variable suffices. There are very very few cases when the doublequotes around variable names are needed.