App\Models\Forum\Post::lastUnreadByUser PHP Method

    public static function lastUnreadByUser($topic, $user)
    {
        if ($user === null) {
            return;
        }
        $startTime = TopicTrack::where('topic_id', $topic->topic_id)->where('user_id', $user->user_id)->value('mark_time');
        if ($startTime === null) {
            return;
        }
        $unreadPostId = $topic->posts()->where('post_time', '>=', $startTime->getTimestamp())->value('post_id');
        if ($unreadPostId === null) {
            return $topic->posts()->orderBy('post_id', 'desc')->value('post_id');
        }
        return $unreadPostId;
    }

 public function show($id)
 {
     $postStartId = Request::input('start');
     $postEndId = get_int(Request::input('end'));
     $nthPost = get_int(Request::input('n'));
     $skipLayout = Request::input('skip_layout') === '1';
     $jumpTo = null;
     $topic = Topic::with('forum.cover')->findOrFail($id);
     $this->authorizeView($topic->forum);
     $posts = $topic->posts();
     if ($postStartId === 'unread') {
         $postStartId = Post::lastUnreadByUser($topic, Auth::user());
     } else {
         $postStartId = get_int($postStartId);
     }
     if ($nthPost !== null) {
         $post = $topic->nthPost($nthPost);
         if ($post) {
             $postStartId = $post->post_id;
         }
     }
     if (!$skipLayout) {
         foreach ([$postStartId, $postEndId, 0] as $jumpPoint) {
             if ($jumpPoint === null) {
                 continue;
             }
             $jumpTo = $jumpPoint;
             break;
         }
     }
     if ($postStartId !== null && !$skipLayout) {
         // move starting post up by ten to avoid hitting
         // page autoloader right after loading the page.
         $postPosition = $topic->postPosition($postStartId);
         $post = $topic->nthPost($postPosition - 10);
         $postStartId = $post->post_id;
     }
     if ($postStartId !== null) {
         $posts = $posts->where('post_id', '>=', $postStartId);
     } elseif ($postEndId !== null) {
         $posts = $posts->where('post_id', '<=', $postEndId)->orderBy('post_id', 'desc');
     }
     $posts = $posts->take(20)->with('user.rank')->with('user.country')->with('user.supports')->get()->sortBy(function ($p) {
         return $p->post_id;
     });
     if ($posts->count() === 0) {
         abort($skipLayout ? 204 : 404);
     }
     $postsPosition = $topic->postsPosition($posts);
     Event::fire(new TopicWasViewed($topic, $posts->last(), Auth::user()));
     $template = $skipLayout ? '_posts' : 'show';
     $cover = fractal_item_array($topic->cover()->firstOrNew([]), new TopicCoverTransformer());
     return view("forum.topics.{$template}", compact('topic', 'posts', 'postsPosition', 'jumpTo', 'cover'));
 }