公式折扣2x1 3x2等PHP

I'm stuck thinking about how to do this: I have a shopping cart and I want to create a coupon system with 2x1, 3x2, 5x3 discount, etc.

But I can't solve this to get a formula and show the total price after apply the coupon.

For example: item price: $5 usd and I have a coupon 2x1:

If I buy 2 items: TOTAL: $5,00 usd (2x1)
If I buy 3 items: TOTAL: $10,00 usd (2x1 + 1)
If I buy 4 items: TOTAL: $10,00 usd (2x1 + 2x1)

In the same way. Item price: $5 usd. and now I have a coupon 3x1.

If I buy 2 items: TOTAL: $10,00 usd (3x1 NOPE!)
If I buy 3 items: TOTAL: $5,00 usd (3x1)
If I buy 4 items: TOTAL: $10,00 usd (3x1 + 1)
If I buy 5 items: TOTAL: $15,00 usd (3x1 + 2)
If I buy 6 items: TOTAL: $10,00 usd (3x1 + 3x1)
If I buy 7 items: TOTAL: $15,00 usd (3x1 + 3x1 + 1)

How to obtain the total price using the coupon in PHP?

Another solution:

function calc($item_count, $unit_price, $coupon)
{
  list($need, $paid) = explode('x', $coupon);
  $left = $item_count % $need;
  return $unit_price * (intval($item_count / $need) * $paid + $left);
}

Haven't tested but this function should work:

function discount($i_boughtitems,$i_necessaryitems,$i_discountitems,$i_priceofitem){
$i_priceofcart = 0;

while($i_boughtitems => $i_necessaryitems){
    $i_priceofcart = $i_priceofcart+($i_priceofitem *$i_necessaryitems);
    $i_boughtitems = $i_boughtitems - $i_necessaryitems;
}
$i_priceofcart =  $i_priceofitem * $i_boughtitems;
return $i_priceofcart;

}