php notice warning id

With $_GET['id'], you are trying to access a variable that comes from the URL query parameter as in product.php?id=123. When the parameter does not exist, you will get the error message.

It is better to reconstruct the code to check if the parameter exists as in

// Check if $_GET ['id'] is set
if (isset ($_GET ['id'])) {
  // Use $_GET ['id']
  $id = $_GET ['id'];
} else {
  // Handle the case when $_GET ['id'] is not set
  // For example, show an error message or redirect to another page
  echo "No id provided";
}