可以'；t使用$this调用数据库连接

每当我试图在Is_Staff中调用$this->dbc时，它都会返回一个错误：

Fatal error: Using $this when not in object context

我不太清楚它为什么要这么做。我试着把$dbc做成全球性的，但似乎没有任何作用。

Class BoardDatabase {
            public $dbhost = "localhost"; // Host
            public $dbuser = "root"; // Username
            public $dbpass = ""; // Password
            public $dbname = "NecBoard"; // Database
            public function __construct($dbhost, $dbuser, $dbpass, $dbname) {
                $this->dbc = new mysqli($this->dbhost, $this->dbuser, $this->dbpass, $this->dbname) or die("Couldnt connect to the database! " . mysqli_errno);
                if(!$this->dbc) {
                    die("Could not connect to database: " . $this->dbc->connect_errno());
                }
            }
            public static function Is_Staff($type, $bool) {
                if($type == "mod") {
                    if($bool == true) {
                        $IsStaff = $this->dbc->prepare("SELECT is_mod FROM users WHERE id = ?");
                        $IsStaff->bind_param("s", $_SESSION["id"]);
                        $IsStaff->execute();
                        $IsStaff->bind_result($is_mod);
                        $IsStaff->store_result();
                        if($IsStaff->num_rows >= 1) {
                            $IsStaff->fetch();
                            if($is_mod >= 1) {
                                return true;
                            } else {
                                return false;
                            }
                        }
                    } else {
                        return false;
                    }
                }
            }
        }