Merge XML Documents with PHP recursive

i have a question about merging xml documents. I need to merge two xml documents to one. The second document have a higher priority, when in the second document are nodes that in the first document not exists.

So at example when i have two documents like this:

1. 
   

   Example

   
   

   
   <root>
   <object3>
    <ObjectX action="show">
       <name>ABC</name>
       <type>XYZ</type>
    </ObjectX>
   </object3>
   
   



2. 
   

   Example

   
   

   
   <root>
   <object1>
       <attr>OK</attr>
       <attr2>200</attr2>
   </object1>
   <object2>
       <item>
           <x>a</x>
           <y>b</y>
       </item>
       <item>
           <x>c</x>
           <y>d</y>
       </item>
       <item>
           <x>e</x>
           <y>f</y>
       </item>
   </object2>
   
   

So when i have this two documents, i need to merge them to one. Some nodes will be handled as complex objects. So the goal that i have is to merge this xml documents to at example (above Examples):

    <root>
    <object1>
        <attr>OK</attr>
        <attr2>200</attr2>
    </object1>
    <object2>
        <item>
            <x>a</x>
            <y>b</y>
        </item>
        <item>
            <x>c</x>
            <y>d</y>
        </item>
        <item>
            <x>e</x>
            <y>f</y>
        </item>
    </object2>
    <object3>
     <ObjectX action="show">
        <name>ABC</name>
        <type>XYZ</type>
     </ObjectX>
    </object3>
</root>

It could happen, that a subobject have already other filled noded in first document, and second have also this nodes but not filled with values. When this happens, should a merging script only merge values wich are not filled with values from filled document.

A Conflict by xml nodes can't happen in the case that i want to implement.

Is there a library for doing this? Maybe for ZF2? Or could anyone else help me out?