In a new post to their site, the PHP.cc (well, Sebastian Bergmann) takes a look at the bytecode caching feature that's included in the latest release of PHP, version 5.5.
PHP is an interpreted language. The default PHP runtime, the one that can be obtained from php.net, compiles PHP sourcecode to an intermediate representation called PHP bytecode which is then executed. A bytecode cache stores this compiled representation of PHP sourcecode in shared memory. This eliminates the need to load and compile sourcecode on each request which leads to a significant increase in performance (up to 70% more requests per second).
He looks some at "the past" of PHP and how it made it up from the world of PHP 4.x to PHP 5.4 and the de-facto bytecode cache before now, APC. With the introduction of PHP 5.5, though, developers have access to an integrated version of the ZendOptimizer+ cache built right into the language. They include a little bit of documentation about how to enable it and how to install it via PECL if it's not already built in.