Question regarding how PHP works internally when running

Hi, I'm trying hard to find out how PHP works internally. Such as, when a request comes into the apache/php engine, what happens next. For e.g. for CGI-BIN using perl, a process will be created for each request and thus is not entirely scalable. Basically, I'm curious of how it handles the scalability and concurrent request (using multithread or a process each). I understand that PHP 4.0 onwards compile and run instead of interpret. This happens all the time by default right? I've tried looking at FAQS, Documentation from php.net but I really couldn't find anything that really addresses this issue. Would appreciate if anyone can point me in the direction where I can find this resource. Thanks. regards, Hong Seng