interface INodeTree
{
   function getParent();
}

class BaseNode implements INodeTree
{
   private $children = array;

   function addChild(BaseNode $child)
   {
      $child = clone $child;
      $child->parent = $this;
      return $children[] = $child;
   }

   function getParent()
   {
       return $this->parent;
   }
}

class NodeType1 extends BaseNode implements INodeTree
{
...
}