php错误中的未定义变量

Question

<?php
    class poll{

        private $db = " " ;
        //private 

        public function __construct($dbConnection){
            $this->$db = $dbConnection;
        }




        public function getPollData(){
            $sql = "SELECT poll_question,yes,no FROM poll WHERE poll_id = 1";

            $statement = $this->$db->prepare($sql);

            $statement->execute();

            $pollData = $statement->fetchData();

            return $pollData;
            }
    }

?>

这是我的代码，当我尝试在我的本地主机上执行时，返回错误如下：注意：未定义的变量：

第8行的/Applications/XAMPP/xamppfiles/htdocs/poll/models/Poll.class.php中的

db

致命错误：无法访问第8行/Applications/XAMPP/xamppfiles/htdocs/poll/models/Poll.class.php中的空属性