How do I pass forward a file or file name from a redirect page

[xxxphp3]

I have a main Php page with 75% constant contents. I want to change the other 25% depending on what page calls the main page.
Example

https://www.example.com/a.html

https://www.example.com/b.html

https://www.example.com/c.html

The above pages use an html redirect code

https://www.example/x/y/z/mainpage.php

Pages a, b, and c are redirect pages to mainpage.php
How can I how can I open storya.txt when someone clicks on ~.com/a in the mainpage.php or storyb.txt if they click on ~ .com/b ?

[Lumpy]

An easy way to do this would be to put a variable into the url. Redirect to something like www.domain.com/whatever?page=a and then in the first few lines of the target page, put in a few lines of php code that will load the right content.

[CODE=php]
<?php
if (isset($_GET['page'])) {

///code to get right content

}
[/CODE]

Simple and effective...goo d luck!