OK, I'll take a stab at this:
If your form has two elements in it:
<form method=post action='page2.php'>
<input type=text name=cost >
<input type=text name=tax>
<input type=submit name=submit>
</form>
then page2.php will have two variables, $cost and $tax (case sensitive) with the values the user entered. If the user enters $100.00 in cost and 8.25% in tax, as far as page2.php is concerned, there is no difference between the form and simply declaring:
$cost = '100.00';
$tax = '8.25';
If you wanted to do anything with the data you could go:
$totalAmt = $cost*($tax/100);
echo $totalAmt;
which would of course yield $108.25 in this case.
If you want to refer only to the form values you can use $HTTP_POST_VARS['cost'] and $HTTP_POST_VARS['tax'] as well, as the $HTTP_POST_VARS is an array containing only the form element values.
Hope this answered your question.