a. It's often html (mostly seen using frames).
b. If youre using QUERY__STRINGS, /somedir/index.php?foo=bar is the same as /somedir/?foo.bar
(not always, but with general basic settings etc.)
c. You could use include's. While on index.php, clicking a link, this will result that you include("secretpage.php"), hence, the address will still state index.php, but the includeed file is "embedded".
// Thomasson