Well, On a properly configured server, the PHP code is never accessable to the visitor - So keeping the 'bounce' pages in an accessable directory is not only pretty secure, its also the easiest way for you to work it.
PHP Syntax is very much like C, so it's pretty simple to pick up. For an example, let's just say you wanted to do it randomly. I know you don't, but it seems like the simplest example I can whip up.
Code:
<?php
if ( rand(1, 2) == 2 )
{
header("location: false.html");
}
else
{
header("location: true.html");
}
?>
That will cause a 50/50 chance to get redirected to one of those pages. The HEADER Command modifies the headers Apache (or whatever webserver you use) sends back to the client. When you set the location argument, you turn the whole page into a redirect. None of this code can be seen by the user access the page - In fact, all they should get back is a set of headers.
You might want to Google for some 'Introduction to PHP' tutorials or something similar. Or check out a 'PHP for Dummies' book from your local library - It gives you a real good start, and you should be able to jump right from there into php.net for more information.