Object model design; base class awareness of derived classes
abstract class AbstractNode{

    protected $_parent;

    public function __construct(self $parent = null){
        $this->_parent = $parent;
    }

    public function get_parent(){
        return $this->_parent;
    }

}

class NodeOne extends AbstractNode{ }

class NodeTwo extends AbstractNode{ }

// more derivatives

public function get_node_one_ancestor(){
    if($this->_parent instanceof NodeOne){
        return $this->_parent;
    }
    if(null !== $this->_parent){
        return $this->_parent->get_node_one_ancestor();
    }
    return null;
}

<root>
    <foo id="1">
        <bar id="2"></bar>
        <bar id="3">
            <foo id="4">
                <bar id="5">
                    <foo id="6">
                        <bar id="7"></bar>
                    </foo>
                </bar>
                <bar id="8"></bar>
            </foo>
        </bar>
    </foo>
</root>

public function get_ancestor($type){
    if($this->_parent instanceof $type){
        return $this->_parent;
    }
    if(null !== $this->_parent){
        return $this->_parent->get_ancestor($type);
    }
    return null;
}