how to make a function in php that discounts a price with a code

if(isset($_POST['submit']) && isset($_POST['discount'])) {

$discount = filter_input(INPUT_POST, "discount");

if(IsInjected($discount))

$errors .= " Bad discount value!";

if($discount == "redund1" || $discount == "refund2") {

$cost -= 5;

var_dump($cost);

$errors .= "A $5 discount has been added to your total";

}

in order form, but cant figure out how to put in pay form for the total.

$final_cost = number_format($cost, 2);

$_SESSION["cost_final"] = $final_cost;

print "

Total: $$final_cost

";

there is an array above that figures out what has been ordered to total that for different options of my product, but i dont know how to get the discount in from my order file to pay file. please help!