Reverse Doubly Linked List Iterator in PHP

Doubly Linked List:

class Node {
   
   public $value;
   public $next;
   public $prev;

   public function __construct($value, $next, $previous) { 
       $this->value = $value;
       $this->next = $next;
       $this->prev = $previous;
   }
}

// Simplified DLink:
class DoublyLinkedList {

   // Member variables for the access points of the list.
   private $head;
   private $tail;
   private $size;

   // Constructor
   public function __construct() {
       $this->head = null;
       $this->tail = null;
       $this->size = null;
   }

   public function isEmpty() {
       return $this->size <= 0;
   }

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

   // Inserts to the end of the list.
   public function addLast($value) {
       if($this->isEmpty()) {
           // If empty, insert and setup.
           $tmp = new Node($value, null, null);
           $this->head = $tmp;
           $this->tail = $tmp;
           $this->size++;
       }
       else 
       {
           $this->tail->next = new Node($value, null, $this->tail);
           $this->tail = $this->tail->next;
           $this->size++;
       }
   }

   public function getIterator(): DLinkIterator
   {
       return new DLinkIterator($this->head);
   }

   public  function getReverseIterator(): DLinkReverseIterator
   {
       return new DLinkReverseIterator($this->tail);
   }
}

Forward Iterator:

class DLinkIterator
{
   protected $current;

   public function __construct($start)
   {
       $this->current = $start;
   }
   
   public function HasNext(): bool
   {
       return $this->current !== null;
   }

   public function Next()
   {
       if(!$this->HasNext())
       {
           throw new Exception("No more nodes to iterate!");
       }

       $val = $this->current->value;
       $this->current = $this->current->next;

       return $val;
   }
}

Reverse Iterator:

class DLinkReverseIterator
{
   protected $current;

   public function __construct($start)
   {
       $this->current = $start;
   }
   
   public function HasNext(): bool
   {
       return $this->current !== null;
   }

   public function Next()
   {
       if(!$this->HasNext())
       {
           throw new Exception("No more nodes to iterate!");
       }

       $val = $this->current->value;
       $this->current = $this->current->prev;

       return $val;
   }
}

Demo:

$myList = new DoublyLinkedList();
$myList->addLast('Hello');
$myList->addLast('World');
$myList->addLast('Goodbye');
$myList->addLast('World');

echo "Forward iterator:\n";
$iter = $myList->getIterator();

while($iter->HasNext())
{
   echo $iter->Next() . "\n";
}

/*
   Output:
       Hello
       World
       Goodbye
       World
*/

echo "Reverse iterator:\n";
$revIter = $myList->getReverseIterator();

while($revIter->HasNext())
{
   echo $revIter->Next() . "\n";
}

/*
   Output:
       World
       Goodbye
       World
       Hello
*/