According to my knowledge, there are many ways to write a PHP application:
1. FastCGI : In this case, the interpreter is loaded during boot time; so the
application itself might also be compiled and loaded once.
2. mod_php : This is same as FastCGI?
3. CGI: The most usual way, there are some PHP code embedded into the web pages, and the PHP interpreter is loaded each time a PHP page is called.
4. application server : The application is compiled and loaded once and the application can either have its own web server, or use a web server as front end, and only handle its own part.