libmongoc's error documentation states that codes for errors in the write concern domain originates from the server. phongo_exception_from_mongoc_domain() should return php_phongo_serverexception_ce for such errors.

This is independent from logic to return BulkWriteException and CommandException from phongo_execute_bulk_write() and phongo_execute_command(), respectively. Those methods will select an appropriate subclass of ServerException.