This useful article shows you what parts of a URL you can build with .htaccess server enviromental variables.

 http://www.quantumwarp.com:8080/subfolder/subfolder2/index.php?turnip=51
 1     2                        3                     4        5         6

- $_SERVER['SERVER_NAME'];  2-3  (port number is not included)
- $_SERVER['REQUEST_URI'];  3-6
- $_SERVER['QUERY_STRING']; 5-6  (? is not included)

- $_SERVER['HTTP_HOST'];    2-3
- $_SERVER['HTTPS'];        This is a Boolean for HTTPS
- $_SERVER['SCRIPT_NAME'];  2-5
- $_SERVER['PHP_SELF'];     2-5

• Anatomy of a URL - very useful article on this subject.