I see why you're setting an error here and then returning valse.  However, this function doesn't ask a question (e.g. - "isDatabaseConfigValid()"), it performs the "validation" action.  With that in mind, this should probably throw an http://www.php.net/manual/en/class.invalidargumentexception.php instead of getting/setting an error.

Having getters/setters for error messages is a red flag that exceptions should be used.