Database

Structure

Notification types:

1. Friend request - friend
2. Friend accepted - friend_accepted
3. Medal received - medal
4. Forum reply - forum_reply
5. Forum mention - forum_mention
6. Comment reply - comment_reply
7. Comment mention - comment_mention
8. Article accepted/declined - article

Feed types:

1. New user - join
2. Level completed - level
3. Friendship formed - friend
4. Medal received - medal
5. Forum thread created - thread
6. Forum post - post
7. Forum karma given - karma
8. Comment - comment
9. Article favourite - favourite
10. Article accepted - article
11. Profile image - image

PHP

PDO

A nice introduction can be found - http://www.phpro.org/tutorials/Introduction-to-PHP-PDO.html