Of course, you could always do this:
<?php
// Check to see if the form has been submitted:
if(isset($_POST))
{
// Set values & do caluclations in here
}
else
{
$loan_balance = '0';
$round_rate = '0';
$smp = '0';
$save = '0';
}
$term = 25; // Define length of payback term
// Theirs is set to 25 years
// Show form here with values from calculations (if any)
?>
I would write the whole thing, but not sure as to exactly how they calculate stuff.