PHP caches = good.
You might want to look at this benchmarks page if you are exploring PHP caches:
Xcache is the latest and greatest open-source PHP cache, and seemingly the most up-to-date and well supported. It's what all the vBulletin kiddies are using nowadays to eke the most performance out of their hosting accounts.
Of those on that list, I've only used eAccelerator (installed in a VPS environment as a RPM = easy, so far trouble free).
You may also want to look at a simple PHP cache called jpcache - http://www.jpcache.com - as well. Unlike some of the others, jpcache is relatively simple to set up and doesn't require the deeper level of system access to install that things like APC and eAccelerator do.
As a general guide to optimizing MySQL/PHP apps, the following is also a good reference I always point people to, even though it is focussed on the Typo 3 CMS.
The page on PHP tuning at:
illustrates very well the virtues of using a PHP cache.