Build a notification system in PHP Codeigniter

I have a web application being developed in PHP using Codeigniter. Basically, it is a requisitions management platform where each user can create requisition which will go through different levels of approvals before it reaches its final stage.

I would like to develop a notification system that work as follows:
- Can be accessed from each controller in my system
- to have a a notification function as follows:

function notify (user_id, rel_to, rel_id, notif_type, msg) { ... }

- user_id: user to notify
- rel_to: string, name of the related table in the databae (for example, related to requisitions table)
- rel_id: id of the record in the related table
- notif_type: notification type (error, success, warning, info, announcement) to be flexible to add more notification types in the future

- to have a notification_viewed function to mark notification as viewed.

- A database table to have at least the following fields:
id (auto incremental) primary key,
user_id (foreign key),
rel_to,
rel_id,
notif_type,
msg,
date_created (auto insert default timestamp),
viewed (false by default)

Notification to be displayed in a notification dropdown menu* as well as in notifications page (see attached images).

* For notification dropdown menu, it'll be better to have a view that generates the content of the dropdown menu in html then we can get the notifications and append them in the html container using Ajax calls.

Please feel free to suggest changes to my structure above in case you feel something is missing or wrong.

Finally, I'll provide you with the theme I'm using in this application.