PHP, Implement a method of a class to append a linked list

Question

I am trying to implement a method of a class to append a linked list of nodes received as an argument at the end of the last element of the invoking object. My code:

<?php

interface INode {
    
    /**
     * Set the data of the node's field
     * 
     * @param mixed $data
     */
    function setData($data);
    
    /**
     * Return the content of the data of this node
     *
     * @return mixed
     */
    function getData();
    
    /**
     * Return the reference to the next node
     *
     * @return object
     */
    function getNext();
    
} // interface INode





interface ILinkedList {
    
    /**
     * Return how many nodes are into the list
     *
     * @return int
     */
    function getCount();
    
    /**
     * Insert a new node at the tail of the list
     *
     * @param mixed $item
     */
    function push($item);
    
    /**
     * Append the given list to this
     * 
     * @param LinkedList $linked_list
     */
    function append(LinkedList $linked_list);
    
    /**
     * Print to video the link list as string like 1->3-> ...
     */
    function printAsList();
    
} // interface ILinkedList





/**
 * Class Node
 */
class Node implements INode {
    
    /**
     * The data of the node
     *
     * @var mixed
     */
    private $_data;
    
    
    
    /**
     * The link to the next node
     *
     * @var object
     */
    private $_next;
    
    
    
    /**
     * The constructor initialize attributes $_data and the link to the next node
     *
     * @param mixed  $data
     * @param object $next
     */
    function __construct($data, $next = NULL)
    {
        $this->_data = $data;
        $this->_next = $next;
    }
    
    
    
    function __toString()
    {
        return "{$this->_data}";
    }
    
    
    
    /**
     * Set the data of the node's field
     * 
     * @param mixed $data
     */
    public function setData($data)
    {
        $this->_data = $data;
    }
    
    
    
    /**
     * Set the reference link to the next node
     *
     * @param object $next
     */
    protected function setNext($next)
    {
        $this->_next = $next;
    }
    
    
    
    /**
     * Return the content of the data of this node
     *
     * @return mixed
     */
    public function getData()
    {
        return $this->_data;
    }
    
    
    
    /**
     * Return the reference to the next node
     *
     * @return object
     */
    public function getNext()
    {
        return $this->_next;
    }
    
} // class Node





/**
 * Class LinkedList
 */
class LinkedList extends Node implements ILinkedList {
    
    /**
     * The pointer of the list
     *
     * @var object|NULL
     */
    private $_head;
    
    
    
    /**
     * Counter of the nodes into the list
     *
     * @var int
     */
    private $_count;
    
    
    
    /**
     * Initialize classe's attributes
     */
    function __construct()
    {
        $this->_head  = NULL;
        $this->_count = 0;
    }
    
    
    
    /**
     * Return the contents of the list as a string
     *
     * @return string
     */
    function __toString()
    {
        return $this->printAsList();
    }
    
    
    
    /**
     * Return how many nodes are into the list
     *
     * @return int
     */
    public function getCount()
    {
        return $this->_count;
    }
    
    
    
    /**
     * Insert a new node at the tail of the list
     *
     * @param mixed $item
     */
    public function push($item) {
        if ($this->_head !== NULL) {
            // Scroll the list
            /** @var Node $curr */
            for($curr = $this->_head; $curr->getNext() !== NULL; $curr = $curr->getNext()) {}
            // Updates the pointer of the last element to the new node
            $curr->setNext(new Node($item));
        }
        else $this->_head = new Node($item);
        
        $this->_count ++;
    }
    
    
    
    /**
     * Append the given list to this
     * 
     * @param LinkedList $linked_list
     */
    public function append(LinkedList $linked_list)
    {
        if($linked_list === NULL) {
            return FALSE;
        }
        
        /** @var Node $curr */
        for($curr = $this->_head; $curr !== NULL; $curr = $curr->getNext()) {}
        
        while($linked_list !== NULL) {
            $curr->setNext(new Node($linked_list->getData())); // <------ Error here
            $linked_list = $linked_list->getNext();
        }
        
        return TRUE;
    }
    
    
    
    /**
     * Print to video the link list as string like 1->3-> ...
     */
    public function printAsList()
    {
        $items = [];
        /** @var Node $curr */
        for($curr = $this->_head; $curr !== NULL; $curr = $curr->getNext()) {
            echo $curr->getData() .'->';
        }
    }
    
} // class LinkedList




$a = new LinkedList;
$a->push(1);
$a->push(2);

$b = new LinkedList;
$b->push(5);
$b->push(6);

$a->append($b); // <---------------- This statement is the one with problems 

?>

For example from the code, I would like after concatenating the object $a should be:

1->2->5->6

The last statement generates the error that I don't know how to correct.

Thanks in advance for any suggestions.