PHP and Apache

We are tring to advocate the use of PHP in some of our projects at work, and had a few questions. When a php file is loaded by Apache is it cached for subsequent accesses or is it destroyed immediately after its first use. I'm asking because we are starting to put together a whole collection of classes for a project we're working on, and do want them to have to all be loaded into memory each time a page is hit. I believe JAVA servelets are loaded and compiled once by each server thread, and we wondered if PHP did the same. Any answers or other suggestions would be welcomed! Thanks! -Matt Phillips