That's quite a mouthful.
Most Web serving involves sending flat files to the user.
This is great, unless you want to let users do something like fill out a user application, or you want every page on your site to have a common look and feel without editing each file by hand. The solutions most people use for these two problems are server-side includes and CGI scripts. PHP solves both these problems. Because it's cross-platform and embedded in HTML, you can use it the same way anywhere it's supported. So, you'd like to see some actual PHP in action.