class animal_factory {

public static function get_animal_obj() {

    switch (trim($_GET['animal'])) {
        case 'monkey':
            require_once self::$sub_plugin_classes_path . 'monkey.php';
            return new monkey();
        case 'sheep':
            require_once self::$sub_plugin_classes_path . 'sheep.php';
            return new sheep();
        case 'bird':
            require_once self::$sub_plugin_classes_path . 'bird.php';
            return new bird();
        default:
            throw new Exception('Unsupported animal class');
    }

  }
}

class animal_factory {

public static function get_animal_obj() {

    $animal = (trim($_GET['animal']));
    require_once self::$sub_plugin_classes_path . $animal . '.php';
    return new $animal();


  }
}