Building circular linked list

Building a circular linked list is not at as hard as it sounds from the name. So far, we have seen that adding a new node at the end is pretty simple; we set the next reference of the last node to NULL. In a circular linked list, the last node's next reference will actually point to the first node, thereby creating a circular list. Let's write a simple circular linked list where the nodes will be inserted at the end of the list:

class CircularLinkedList { 

    private $_firstNode = NULL; 
    private $_totalNode = 0; 

    public function insertAtEnd(string $data = NULL) { 
        $newNode = new ListNode($data); 
        if ($this->_firstNode === NULL) { 
            $this->_firstNode = &$newNode; 
        } else { 
            $currentNode = $this->_firstNode; 
            while ($currentNode->next !== $this->_firstNode) { 
                $currentNode = $currentNode->next; 
            } 
            $currentNode->next = $newNode...