I suppose what you need are:
-) $_REQUEST parameters (e.g. /index.php?site=news or /index.php?site=guestbook), google how to use them.
-) switch - case constructs (again use google)
-) and finally, includes, typically include("myscript.html") or include_once("myscript.html"), again google
These steps should help you!
Take a look at this tutorial, I learned PHP myself using this one 10 years ago: