Question

I’m trying to figure out what exceptions should be used for; logging application errors, or displaying user errors, and if both, how should I distinguish between and handle a user error (e.g. failed login) and an application error (e.g. a failure to connect to the database) with exceptions.

For example, let’s say a user attempts to login, but fails (user error):

if(!$user->login($_POST)) {
    //$user->login() throws an Exception, throw new Exception("failed login"), on fail.
    catch(Exception $e) {
      print $e->getMessage();
    }
}

Or perhaps a database error occurs (error logging):

public function query($query) {
$this->resource = mysql_query($query) or throw new Exception("Database error:" . mysql_error());
}

Which of the above two examples would be an appropriate use case, if either?

Answer 1

Exceptions are for signalling exceptional behaviour on part of your program. During normal operation, your program should not usually throw exceptions at all. Something like a failed login is a normal result of your login mechanism, not an exception.

_{[A failed login may be an exception on the user‘s end (if the user had reason to expect the login to work unconditionally)!]}

Typical exceptional situations are running out of memory, running out of disk space, an error in the database connection, or an error during reading or writing a file. All those are things that you would normally expect to work, so an error there is an exceptional condition.

See my previous answer for more details.