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;