Auto increase value php in file based on number entered - php

Question

I have 5 different variables that I need to calculate. Right now these work, but what I want to do is have the price recalculate every 50 increase. Now, I can code into the form to only allow a maximum purchase of 50 at a time, but I want to allow for as many of each as the person has money for (that is what the numbers are). So I think what I need is a parabola style formula that auto increases every 50, but I don't know exactly what I'm looking for or how to implement it.
$resprice = 20000+($cityFetch['residential']*502)+($cityNum*1000);
$comprice = 18000+($cityFetch['commercial']*506)+($cityNum*1000);
$indprice = 23000+($cityFetch['industrial']*508)+($cityNum*1000);
$landprice = 600+($cityFetch['land']*.008);
$milprice = 25000+($cityFetch['military']*512)+($cityNum*1000);
EDIT: I was indicated that a loop will work for this.
EDIT3: Finally landed on this, Havvy helped me out on IRC
$cityCheck = mysql_query("SELECT * FROM cities WHERE nation='$nation'");
$cityNum = mysql_num_rows($cityCheck);
function determinePrice($baseCost, $scalar, $current, $bought) {
$min = $baseCost + $scalar * ($current + 1) + ($cityNum * 1000);
$max = $baseCost + $scalar * ($current + $bought) + ($cityNum * 1000);
$average = ($min + $max) / 2;
return $average * $bought;
}
$resprice = determinePrice(20000, 502, $cityFetch['residential'], $cityFetch['residential']);
$comprice = determinePrice(18000, 506, $cityFetch['commercial'], $cityFetch['commercial']);
$indprice = determinePrice(23000, 508, $cityFetch['industrial'], $cityFetch['industrial']);
$milprice = determinePrice(25000, 502, $cityFetch['residential'], $cityFetch['military']);
$landprice = 600+($cityFetch['land']*.008);

Answer 1

I use a step size of 1, instead of 50. It's more logical (one less magic number) to not give discounts based on how much you buy at a time, and it would be confusing to new players having a stepwise-linear function.
The reasoning behind this function can be found at http://betterexplained.com/articles/techniques-for-adding-the-numbers-1-to-100/.
function determinePrice($baseCost, $scalar, $current, $bought) {
$min = $scalar * ($current + 1) + ($cityNum * 1000);
$max = $scalar * ($current + $bought) + ($cityNum * 1000);
$average = ($min + $max) / 2;
return $average * $bought;
}
Using it would look something like this:
$resprice = determinePrice(20000, 502, $cityFetch['residential'], $resBought);
It won't work for land though, because land doesn't include the city price in it. For land, you can copy the function and remove the $cityNum * 1000 portions. You could also add in a $isLand parameter to the signature, and use a ternary operator to add 0 instead.
If you don't like that $cityNum has to already be declared with the proper value before declaring this function, you can always add it to the function signature, making it a pure function.

Answer 2

$count=1
if($count==50)
{
You increase the prices then
$count=1;
}
$resprice = 20000+($cityFetch['residential']*502)+($cityNum*1000);
$comprice = 18000+($cityFetch['commercial']*506)+($cityNum*1000);
$indprice = 23000+($cityFetch['industrial']*508)+($cityNum*1000);
$landprice = 600+($cityFetch['land']*.008);
$milprice = 25000+($cityFetch['military']*512)+($cityNum*1000);
$count++;