Php caching? or database end?

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http://ubuntuforums.org – I have a site that would pretty much looked like a forum and the topic threads are in bold. So what i want is when a specific user visit's that thread, the next time he will see its title it will be unbold unless a new user replied to that thread. like in this forums. any suggestions on how to do it? or algorithms? I would appreciate if you have so. I'm new to web developing. thanks. (Hardware)