我想在PHP中显示同一类和差异函数中的表行

问：

如何在PHP中显示“我的用户”表中的行 arr（） 函数无法连接数据库

class DB {
    protected $db_server = 'localhost';
    protected $db_username = 'root';
    protected $db_password = 'root';
    protected $db_name = 'dbname';
    public function connect() {
        $connect_db = new mysqli($this->db_server, $this->db_username, $this->db_password, $this->db_name);
        if(mysqli_connect_errno()) {
            printf("Connection failed!", mysqli_connect_error());
            exit();
        } else {
            echo "connected";
        }
        return true;
    }
    function arr() {
        $conn = $this->connect();
        if($query = mysqli_query($conn, "SELECT * FROM users")) {
            echo 'row is ' . mysqli_num_rows($query);
        }
    }
}
$db = new DB();
$db->arr();

结果 -> “连接的行是 ” 而不是计算行数