Caching For A PHP / MYSQL Website

We require someone to suggestion an implement a basic but suitable caching system for our website. The website is a PHP Website and everything is stored in MYSQL Database. It is a real-time interaction website whereby users ask question and give answers as well. Quasi a replica of Yahoo! Answers. What type of cache would you suggest to implement? Please include this information in your bid in order to be seriously considered. The MYSQL results are already cached and the website has already been optimized to the max. We need to reduce to load to server now by using something basic such as this: [login to view URL] I am not sure if this is suitable since the website will have constant posting of question and answers. The suggested caching system should of course be implemented by the winning bidder. Thanks in advance to everyone!