php : reverse a linked list

reversedLinkedList.php

<?php

class Node {
    
    public $data = null;
    public $next = null;
    
    public function __construct(string $data = null){
        $this->data = $data;
    }
}

class NodeList {
    
    private $firstNode  = null;
    private $count = 0;
    
    public function insert(string $data = null){
        $newNode = new Node($data);
        
        if($this->firstNode === null){
             $this->firstNode = $newNode;
        }else{
            
            $current = $this->firstNode;
            while($current->next !== null) {
              
              $current = $current->next;
            }    
          
            $current->next = $newNode;
        }
       
        $this->count++;
    }
    
    public function display(){
        echo "Total nodes : " . $this->count . "\n";
        
        $current = $this->firstNode;
        while($current !== null){
            echo $current->data . "\n";
            $current = $current->next;
        }
    }


    public function reverse() {
        if ($this->firstNode !== null && $this->firstNode->next) {
           
            $currentNode  = $this->firstNode;
            $reversedList = null;
            $next = null;
            
            while ($currentNode !== null) {
                
                $next = $currentNode->next;
                
                $currentNode->next = $reversedList;
                $reversedList = $currentNode;
                
                $currentNode = $next;
            }
            
            $this->firstNode = $reversedList;
        }
    }
}

$nodeList = new NodeList();

// insert 3 nodes 6 , 2 , 3
$nodeList->insert(6);
$nodeList->insert(2);
$nodeList->insert(3);

// display list's nodes
$nodeList->display();

// reverse
$nodeList->reverse();

// check if reversed
$nodeList->display();