Now that is where you Job is started :) Its now up to you about how you can handle these.

Okay, so I have fixed the double appearance.
I now have it looking pretty cool...
The last thing to conquer is it actually doing what it says it does, and also not take the user to a 'page not found' error. But, it's getting there!

:D

Well 404 means URL didn't exists and that make sense there is no such a URL exists. Regarding double appearance.

I don't understand also why $version == "6.0" and 5.6 have different URLS? I didn't meant 6.0 codes to these I mean the notification types I mentioned are some from 6.0

You need to check if you didn't added that component twice in the system or it didn't exists twice in components table.

@Arsalan, Thank you!!!

I have the code in a new component, tentatively dubbed "MarkNotification". I'm encountering a couple of things, that I didn't expect.
First: it pops it in twice. (I have no idea why, but...
And secondly, both the code that you gave me, and recreating the notification's code both redirect to 404 error: page not found. It does mark it -- as the badge number goes down; but instead of refreshing to where they were (REF) or home ('home') it always goes to a 404 error page.
And the delete doesn't delete...
In the image, "Mark ✔️" marks the notification, and "❌ " deletes the notification. (Supposed to, anyways.)

Code:

 $version = $Ossn->siteSettings->site_version; //may be off, because hand writing it here from memory; Uses the code from getsettings function.
 $location = urlencode(ossn_site_url('home'));
          ... 
      if ($version <= "5.6") {
            $frag->appendXml("<a href='notification/read/{$notification_guid}?notification={$location}' style='position:relative;float:right;' title='{$notif_title}'>Mark ✔️</a>");
      } elseif ($version == "6.0") {
            $frag->appendXml("<a href='read?guid={$notification_guid}' style='position:relative;float:right;' title='{$notif_title}'>Mark ✔️</a>");
      } else {
            return; // exit if not an appropriate version.
      }
      $div->appendChild($frag);

Thoughts?
-Michieal.

Create a new component and add following in your ossn_com.php file

<?php
function ossn_notifications_custom_init(){
        $alltypes = array(
                'like:annotation',
                'like:entity',
                'comments:post',
                'ossnpoke:poke',
                'like:entity:file:profile:photo',
                'comments:entity:file:profile:photo',
                'like:entity:file:profile:cover',
                'comments:entity:file:profile:cover',
                'wall:friends:tag',
                'like:post:businesspage:wall',
                'comments:post:businesspage:wall',
                'event:relation:going',
                'event:relation:interested',
                'event:relation:nointerested',
                'comments:entity:event:wall',
                'like:post:businesspage:wall',
                'like:entity:event:wall',
                'like:annotation',
                'groupinvite',
                'group:joinrequest',
                'like:annotation:comments:post',
                'like:annotation:comments:entity',
        );
        foreach ($alltypes as $type){
                ossn_add_hook('notification:view', $type, 'notificaiton_insert_custom_html');
        }
}
function notificaiton_insert_custom_html($hook, $htype, $return, $params){
        $notification_guid = $params->guid;
        if(!empty($return)){
                $doc = new DOMDocument();
                $doc->loadHTML($return, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);

                $div = $doc->getElementsByTagName('a')->item(0);

                $frag = $doc->createDocumentFragment();
                $frag->appendXml("<a href='read?guid={$notification_guid}'><input type='radio' /></a>");
                $div->appendChild($frag);

                return $doc->saveHTML();
        }
        return $return;
}
ossn_register_callback('ossn', 'init', 'ossn_notifications_custom_init');

Right now each notification type template is handled via component file. The only way is to write a notification:view hooks again for each type but you can avoid that by above code just insert a new tag for each notification template.

NOTE The types will change in OSSN 6.0 and I tried to include new types from v6.0 in above code also.

You can adjust the style and position the element as per your need.

Hello Michieal~Coder~ . Can u provide me your email address? I have some queries regarding attachments in 1:1 messages. I can pay you if you can solve those issues. My email id is : [email protected]