You actually don't need to do any snippet of code to set session Message , Fuel php provides a very good interface to handle such feature . Its the core Session Class here is how we need to use as per documentation .
in your case lets say you need to redirect to login_action from register_action and after successful registration process , then just set the flash message like below
Session::set_flash('success', 'Your account has been successfully created !');
Later in the view file of the login you can just use the below code to get the flash message that you want . some what like this
<?php if (Session::get_flash('success')): ?>
<div class="alert-box info">
<?php echo Session::get_flash('success') ; ?>
</div>
<?php endif; ?>
Also You can delete the session message say 'success' in the above example by just using delete_flash method like below
Session::delete_flash('success');
To know more about session class you can learn from the link below
FuelPhp Session Class docuementation
And also to redirect the user you don't need to write any new PHP code, Fuel php gives a nice interface for that too , its the 'Response' class which has methods defined for you to do that
Here is how you need to redirect the user in any of your controller action
Response::redirect('yourRouteName');
Its always a good practice to exploit the classes defined in the framework , which are actually defined with an intention to make the developers life easy .
Being object oriented in your programming approach especially when using a good quality OOP MVC framework like fuelPhp prevents you from reinventing the wheel always ( DRY ) by providing you access to ready made Getters and setters .
Hope this helps