Read http://www.php.net/manual/en/function.include.php so that you understand how the function works. The purpose of include() is to allow you to (a) break your program up into logical components so that it can be easily maintained, and (b) allow components to be shared among multiple programs, so that you are not reinventing the wheel every time you write a script.
Read http://hoohoo.ncsa.uiuc.edu/cgi/ so that you understand the fundamentals of CGI and dynamic pages. Although most PHP installations are server modules and not CGI, the CGI conventions are used to pass information (such as form submissions) to scripts. Be sure you understand the difference between GET and POST.
From the NCSA documentation:
GET -- this is the default method and causes the fill-out form contents to be appended to the URL as if they were a normal query.
POST -- this method causes the fill-out form contents to be sent to the server in a data body rather than as part of the URL.
