(PHP 5, PHP 7)
DOMNode::replaceChild — Replaces a child
   This function replaces the child oldnode 
   with the passed new node. If the newnode is already a child it 
   will not be added a second time. If the replacement succeeds the 
   old node is returned.
  
newnodeThe new node. It must be a member of the target document, i.e. created by one of the DOMDocument->createXXX() methods or imported in the document by DOMDocument::importNode.
oldnodeThe old node.
   The old node or FALSE if an error occur.
  
DOM_NO_MODIFICATION_ALLOWED_ERRRaised if this node is readonly or if the previous parent of the node being inserted is readonly.
DOM_HIERARCHY_REQUEST_ERR
       Raised if this node is of a type that does not allow children of the 
       type of the newnode node, or if the node to 
       put in is one of this node's ancestors or this node itself.
      
DOM_WRONG_DOCUMENT_ERR
       Raised if newnode was created from a different 
       document than the one that created this node.
      
DOM_NOT_FOUND
       Raised if oldnode is not a child of this node.