I notice when my forum pages come up on Yahoo or Google search it looks like this Free! The Logistic Forums - Join Now - :: View topic - Guests who are ... Forum Header is Free! The Logistic Forums - Join Now - Next bit is View topic which I have no idea where that comes from. Post Subject is Guests who are which is cutoff and no-one really knows what the subject is. It looks like the Forum Header gets 35 characters and the Post Subject gets around 15 which means all of my subjects are unreadable. I notice other forums like this one have the subject first using 35 characters and the header last. How can I change mine to be more readable?
This is rather SEO than PHP. Try to set your forum to output descriptive and preferably unique meta description tags, for Google it often works. Also make sure your title tag is descriptive. These are a must if you care for your SE performance ;-)
I notice google links for other forums like this one show the subject in the first part instead of the last part like mine so the subject is more descriptive and likely to get more traffic.
Depending on the forum, there should be some settings for this that you have to enable or got an add-on that will automatically insert the meta description tag. The content you have posted comes from withing the body tag if description is not available.