How to subtract db values using php

How to subtract db values using php

How exactly do I subtract values from a database using php? I see plenty
of examples using static variables such as
<?php
$first_number = 10;
$second_number = 2;
$third_number = 3;
$sum_total = $third_number + $second_number * $first_number;
print ($sum_total);
?>
However I'm looking to subtract one database value from another, then
multiply that value by another db value. To give some more detail, I have
an inventory database where I'm echoing the values into a table, I'm
attempting to subtract the total quantity of an item from the minimum
quantity, to see how many need to be ordered, then multiply the number of
parts we need to order by the cost of that part. I've dug around and found
a few possible methods such as
$query = "SELECT `db`,
(`minimumquantity` - `totalquantity`) AS `quantitytoorder`
FROM `db`
WHERE id ='".$id."';"
and
<?php
$minimumquantity = $_GET['minimumquantity'];
$totalquantity = $_GET['totalquantity'];
$quantitytoorder = $minimumquantity - $totalquantity;
print ($quantitytoorder);
?>
Please before you laugh, I'm very much a beginner, can anyone point me in
the right direction, or provide me with proper examples? My only real
resource is the net and most examples I find are very high-level.