سلام کد دقیقا مثل استاد نوشتم ولی در اجرا نمیکنه

$sql = "SELECT * FROM people where age < 20";
$stmt = $db->prepare($sql);
$stmt->execute();
$rows = $stmt->fetchAll();
mydump($rows);
error me
PHP Warning:  Undefined variable $db in C:\\xampp\\htdocs\\mysql\\mysqli\\select1.php on line 5
Warning: Undefined variable $db in C:\\xampp\\htdocs\\mysql\\mysqli\\select1.php on line 5
PHP Fatal error:  Uncaught Error: Call to a member function prepare() on null in C:\\xampp\\htdocs\\mysql\\mysqli\\select1.php:5
Stack trace:
#0 {main}
  thrown in C:\\xampp\\htdocs\\mysql\\mysqli\\select1.php on line 
Fatal error: Uncaught Error: Call to a member function prepare() on null in C:\\xampp\\htdocs\\mysql\\mysqli\\select1.php:5
Stack trace:
#0 {main}
  thrown in C:\\xampp\\htdocs\\mysql\\mysqli\\select1.php on line 5