UnEarth is a library for PHP which helps web developers speed up
web pages by caching them.
With this library, you can cache not only whole pages but smaller subsections according to different requirements. You could call this "smart caching," so when each page is rendered PHP needs only execute the code it needs to while the rest is drawn from memory.
See the examples page to get an idea of how it is used. It's meant to be simple to implement.
SourceForge project page