PHP Linked List with end and start List Node pointers

<?php

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

class LinkedList
{
    private $_firstNode = NULL;
    private $_lastNode = NULL;
    private $_totalNodes = 0;
    
    function insert(string $data = NULL)
    {
        $newNode = new ListNode($data);
        
    	if($this->_firstNode === NULL){
    	  $this->_firstNode = &$newNode;
    	  $this->_lastNode = $this->_firstNode;
    	  $this->_totalNodes++;
    	  return true;
    	} 
    	
    	$curentNode = $this->_firstNode;
    	
    	while($curentNode->next !== NULL){
    	    $curentNode = $curentNode->next;
    	}
    	
    	$curentNode->next = $newNode;
    	$this->_lastNode = $newNode;
    	
    	$this->_totalNodes++;
    	return true;
    	
    }
    
    function printData(){
        $pointer = $this->_firstNode;
        if(!$pointer) return;
        
        while($pointer != NULL){
            
            echo $pointer->data . PHP_EOL;
            
            $pointer = $pointer->next;
        }
    }
    
    function returnListSize(){
        return $this->_totalNodes;
    }
    
    function search($data = NULL){
        if($this->_totalNodes){
            $currentNode = $this->_firstNode;
            while($currentNode !== NULL){
                if($data == $currentNode->data){
                    return $currentNode;
                }
                
                $currentNode = $currentNode->next;
            }  
        }

        return FALSE;
    }
    
    function insertBefore($data = NULL, $search = NULL){
        $newNode = new ListNode($data);
        if($this->_firstNode){
            $currentNode = $this->_firstNode;
            $prevNode = NULL;
            
            while($currentNode !== NULL){
                
                // insert before first node
                if($this->_firstNode->data == $search){
                    $newNode->next = $currentNode;
                    $this->_firstNode = $newNode;
                    
                    $this->_totalNodes++;
                    return true;
                }
                
                // insert between nodes
                if($currentNode->data == $search){
                    $prevNode->next = $newNode;
                    $newNode->next = $currentNode;
                    
                    $this->_totalNodes++;
                    return true;
                }
                
                $prevNode = $currentNode;
                $currentNode = $currentNode->next;
                
            }
        }
        
        return false;
    }
    
    function insertAfter($data = NULL, $search = NULL){
        $newNode = new ListNode($data);
        if($this->_firstNode){
            
            $nextNode = NULL;
            $currentNode = $this->_firstNode;
            
            while($currentNode !== NULL){
                // insert as last position
                if($currentNode->next == NULL){
                    $currentNode->next = $newNode;
                    $this->_lastNode = $currentNode->next;
                    
                    $this->_totalNodes++;
                    return true;
                }
            
                // insert after (between nodes)
                if($currentNode->data == $search){

                    $nextNode = $currentNode->next;
                    $newNode->next = $nextNode;
                    $currentNode->next = $newNode;
                    
                    $this->_totalNodes++;
                    return true;
                }
                
                // NOTE! The order do mater
                $currentNode = $currentNode->next;
                $nextNode = $currentNode->next;
                
            }
        }
        return false;
    }
    
    public function removeFirst(){
        if($this->_firstNode !== NULL){
            $this->_firstNode = $this->_firstNode->next;
            $this->_totalNodes--;
            return true;
        }else{
            return false;
        }
    }
    
    public function removeLast(){
        if($this->_lastNode !== NULL){
            
            $currentNode = $this->_firstNode;
            $prevNode = NULL;
            
            if($this->_firstNode === $this->_lastNode){
                $this->_firstNode = $this->_lastNode = NULL;
                $this->_totalNodes--;
                return true;
            }
            
            while($currentNode !== NULL){
                
                if($currentNode->next === $this->_lastNode){
                    $currentNode->next = NULL;
                    $this->_lastNode = $currentNode;
                    $this->_totalNodes--;
                    return true;
                }
                
                $prevNode = $currentNode;
                $currentNode = $currentNode->next;
                
            }

        }else{
            return false;
        }
    }
}

Testing PHP linked list

$l = new LinkedList();
$l->insert("Wiktor");
$l->insert("Wojtuś");
$l->insert("Kubuś");


$l->insertBefore("Justyna", "Wiktor");
$l->insertBefore("Kotek", "Justyna");
$l->insertAfter("Konrad", "Kubuś");

var_dump($l->removeFirst());
var_dump($l->removeLast());

$l->printData();

echo $l->returnListSize(). PHP_EOL;

echo ($l->search("Wiktor"))->data. PHP_EOL;

var_dump($l);