How To Display Default Text In PHP Include?

Guys, I want to display default text in php include!

Example:
<?php
include 'default.php';
?>
when my user submit a form using post method there may should display
<?php
include 'submit.php';
?>
instead of 'default.php'

i want to use 'submit.php' while submiting a form using Post method

sample tag: <form action="/home.php" method="post">
.....
<input type="hidden" name="any" value="submit.php">
<input name="doit" type="submit" value="Submit"/></form>

can u please answer me kindly