A simple Linked List in PHP

LinkedList.php

<?php

class Node {

    public $data;
    public $next;

    public function __construct($data)
    {
        $this->data = $data;
        $this->next = NULL;
    }

    public function getData()
    {
        return $this->data;
    }

}

class LinkedList { // REMEMBER, it's NOT a queue: each node is inserted as firstNode


    private $firstNode;
    private $lastNode;

    public function __construct()
    {
        $this->firstNode = $this->lastNode = NULL;
    }

    public function isEmpty() {
        return $this->firstNode == NULL;
    }

    public function emptyList() {
        $this->__construct();
    }

    public function insertNode($data) {

        $new_node = new Node($data);
        $new_node->next = &$this->firstNode;
        $this->firstNode = &$new_node;
        if($this->lastNode == NULL) $this->lastNode = &$new_node;

    }

    public function deleteNode($data) {

        $current = $previous = $this->firstNode;

        while ($current != NULL) {

            if($current->getData() == $data) {

                $previous->next = $current->next;

                if($current == $this->firstNode) $this->firstNode = $current->next;
                if($current == $this->lastNode) $this->lastNode = $previous;

            }
            $previous = $current;
            $current = $current->next;

        }
    }

    public function traverseList() {

        $current_node = $this->firstNode;
        while($current_node != NULL) {
            echo $current_node->getData().'->';
            $current_node = $current_node->next;
        }
        echo "NULL\n";
    }

    function reverseList() {

        if($this->firstNode != NULL) {
            if($this->firstNode->next != NULL) {

                $current = $this->firstNode;
                $new = NULL;
                while($current != NULL) {
                    $temp = $current->next;
                    $current->next = $new;
                    $new = $current;
                    $current = $temp;
                }
                $this->firstNode = $new;
            }
        }
    }


}



$ll = new LinkedList();
$ll->insertNode(3);
$ll->insertNode(2);
$ll->insertNode(4);
$ll->insertNode(5);
$ll->traverseList();

$ll->deleteNode(3);
$ll->traverseList();

$ll->reverseList();
$ll->traverseList();

there's a problem in deleteNode method please fix it
or tell me what is the solution