I'm not sure of the best way to attack this, I'm just simply pointing out that I feel there is a an area that has an opportunity for improvement. As for the unanswered threads tool, while great, will exclude items such as mine that I provided as an example.
The PHP forum is very large and very active, a lot of days, a post will be pushed on to page 2 within the day of creation and will fall out of view to a lot of people. PHP is also a very broad topic with many specializations I feel. If it were to be broken up into smaller specializations (OOP? Frameworks?) then these categories may become less used / active than we are used to, but would take care of issue one, as well as bring attention to newcomers that there are better approaches / methods past what they are using now.
Though I know breaking forums up further is frowned upon, but thinking on this further, SP is THE premiere source the web dev community, and we are just lumping its primary server side language all together in one group? I'm not sure how it should be split up, but I feel it's activity warrants it.
Though [SOLVED] at the beginning of a thread name is useful, I think it's a bit of an eyesore, and we can also do better than that if we go down that road (which I think would prove most useful). Color code the thread row based on it's status perhaps? Open, Solved and Inactive (30 days of inactivity without being flipped to "solved" by the thread creator) but without manipulating the thread title... Quick toggle to filter between these 3 in the thread browser would bring in the ones that might still need attention. Just some ideas..