According to the Alternative PHP Cache (APC) Web site:

"APC was conceived of to provide a way of boosting the performance of PHP on heavily loaded sites by providing a way for scripts to be cached in a compiled state, so that the overhead of parsing and compiling can be almost completely eliminated."

Consider the use of the Alternative PHP Cache (APC) to help improve the performance of your Web application. It is open-source and free, and could loosely be described as an open-source competitor to the Zend Optimizer (discussed in Considering the use of the Zend Optimizer).