您正在使用$conn->，就像使用$db进行连接一样，因此下面的代码段

You are using $conn-> where as you are using $db for connection so the below snippet

$conn ->prepare($query);

应写为

$db->prepare($query);

此外，您从哪里获得的$exception?您的catch语句应类似于

Also, where did you got $exception from? Your catch statement should look like

catch(PDOException $exception) {

}