Identifying between PHP Internal Classes and User-defined Classes

I've been trawling through the PHP docs trying to identify if there is any method that will allow me to differentiate between instances of a built-in class (such as DateTime or PDO) and User-defined Classes, but without any success.

The only approach that I have found so far is to try and bind a closure to the instance. When doing this with a built-in class, it displays a warning (yuk) and returns a null.

$targetObject = new DateTime();

$closure = function();

$test = @$closure->bindTo($targetObject, get_class($targetObject));
if ($test === false) {
    throw new Exception('General failure');
} elseif ($test === null) {
    throw new Exception('Unable to bind to internal class');
}

Is anybody aware of a cleaner approach to this problem?