How to create a notification menu

[programmer]

I wrote a forum app sometimes ago and am trying to improve it . I have an issue that is hard for me to solve(I don’t even know how to solve it, I just have the idea), how can I create a notification menu like the one on this site, stackoverflow etc that display at the top of the menu for a user about messages that were received on thread or post he start or comment on.

The forum was created using PHP

[Andres_Vaquero]

Hi there, here’s a basic class I have written in 5 minutes just for you
Please note I have not tested it but it should give you an idea and set you in the right direction. If you have any questions please ask:

<?php

class Notifications 
{

    public function __construct(string $id) 
    {
        $this->id = $id;
        if(session_id() == '' || !isset($_SESSION)) {
            session_start();
        }      
    }

    public function getMessages() {
        if (!empty($_SESSION['notifications'][$this->id])) {
            return $_SESSION['notifications'][$this->id];
        }
        return [];
    }

    public function render() {
        $rv = "<ul id='$this->id-notifications' class='notifications'>";
        foreach ($this->getMessages() as $message) {
            $rv .= $this->renderMessage($message);
        }
        $rv.= "</ul>";
        return $rv;
    }
    
    public function renderMessage(array $message) {
        $type = $message['type'] ?? 'info';
        if (empty($message['text'])) {
            return;
        }
        $rv .= "<li class='message $type-message'>";
        $rv .= $message['text'];
        $rv .= "</li>";
        return $rv;
    }

    public function addMessage(string $text, string $type) {
        $_SESSION['notifications'][$this->id][] = [
            'text' => $text,
            'type' => $type
        ];
    }

    public static function addMessageStatic(string $text, string $type, string $notificationsId) {
        $_SESSION['notifications'][$notificationsId][] = [
            'text' => $text,
            'type' => $type
        ];
    }
}

$notifications = new Notifications('myNotifications');
$notifications->addMessage('Hello World!');
Notifications::addMessageStatic("Hello World, I cannot reach the instance of my class but I know the id!", 'info', 'myNotifications');
echo $notifications->render();