用 PHP 实现一个双向队列

class DoubleQeque{
    private $queue = array();
    public function addFirst($item){
        return array_unshift($this->queue, $item);
    }
    public function addLast($item){
        return array_push($this->queue, $item);
    }
    public function removeFirst(){
        return array_shift($this->queue);
    }
    public function removeLast(){
        return array_pop($this->queue);
    }
}