PHP mt_random in a loop from WP ALL Import for Woocommerce, Wordpress

Question

I use WP All Import, and I use this code to exchange the course. I would like to make a discount randomly at the same time after converting it to my currency.

This is the code

function add_exchangerate($x) {
  $markup = 20;
  $y = $x * $markup;
  return $y;
}

What can I add to it to make it, for example, every 20th time that it launches this function, it will make a discount, maybe 13%?

thought about adding

$i = 1;
$i++
$xnum = mt_rand(1,1000)
while ($x = $xnum) {
$z = $x/1.15
return $z
}

Answer 1

Use a session to store the counter, the rest of the logic is pretty simple

function add_exchangerate($x)
    {
    $_SESSION['calladd'] = isset($_SESSION['calladd']) ? $_SESSION['calladd'] + 1 : 0;
    if ($_SESSION['calladd'] == 20)
        {
        $discount = 13/100;
        $_SESSION['calladd'] = 0;
        }
    else
        {
        $discount = 1;
        }

    $markup = 20;
    $y = $x * $markup;
    return $y * $discount;
    }