simple solution:
in index.php, place a code which detects whether the thynk-you page is desired. if so, display "thank you" html and exit() (or redirect somewhere else).
(note: include the if($GET['f'] == 'submit') part after_ eventual form processing or it won't be processed)
index.php before:
<?
include "...";
...
after:
<?
if($_GET['f'] == 'submit')
{
echo "<html><body>Thank you!</body></html>";
exit;
}
include "...";
...
or:
<?
if($_GET['f'] == 'submit')
header("Location: thankyou.html");
include "...";
...