PHP forum help

[Monster Killer]

Hello.

I am designing a php forum but one of the things i can't seem to work out or find is how you do the icons on the left side such as 'New' 'No new posts' etc.

What i can't work out about it is how you tell php if there is a new post in topic that the user had not seen or if the if there is no new posts that the user wouldn't have seen. if you understand what im on about.

Any help is much appreciated.

[Monster Killer]

So basically. I want to get the read/unread status of the thread for the user.