php linked list queue

Example code:

class Queue{

  private $last;
  private $first;
  private $oldfirst;
  private static $n=0;

  public function __construct(){
    $this->last   = null;
    $this->first  = null;
    $this->oldfirst = null;
  }

  public function push($item){
    $this->oldfirst = $this->last;
    $this->last = new Node();
    $this->last->item = $item;
    $this->last->next = null;
    if(empty($this->first)){
      $this->first = $this->last;
    }else{
      $this->oldfirst->next = $this->last;
    }
    self::$n++;
  }

  public function pop(){
    if(self::$n<0){
      return null;
    }
    $item = $this->first->item;
    $this->first = $this->first->next;
    self::$n--;
    return $item;
  }

}

class Node{
  public $item;
  public $next;
}

$Queue = new Queue();
$Queue->push("a");
$Queue->push("b");
$Queue->push("c");
echo $Queue->pop().PHP_EOL;
echo $Queue->pop().PHP_EOL;
echo $Queue->pop().PHP_EOL;
echo $Queue->pop().PHP_EOL;

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