Calculation error, help me find please? - php

Question

I am running subscriptions on my website.
I have a 1,3,6 and 12 months of subscription, and I would like the user to be able to change that subscription whenever they feel like.
However, I need to calculate the amount of money the user had to pay had he or she signed up for the shorter term, rather the relatively cheap,longer one.
I made this function optimized_subscription_total($active_sub_time,$arr_sub_values) so that it returns that sum of money exactly.
<?php
function optimized_subscription_total($active_sub_time,$arr_sub_values)
{
// This function takes a row from the DB where prices for each period of time usage is listed. there are prices for 1 month, 3 months,6 and 12 months.
// when the user has subscribed for 12 months, and the user asks for a refund, after they used 9 months and 6 days for example, the system treats the refund as if they subscribed for (in months) COST_6 + COST_3 + (COST_1/30)*6
// the result of the function is then subtracted from the amount they actually paid and is considered the refund.
// $arr_sub_values is the associative row from the DB, containing the prices
// $active_sub_time is measured in months and is a double
$result=0;
while(($active_sub_time-12)>=0)
{
$active_sub_time-=12;
$result+=($arr_subscription_values['COST_12']);
}
while(($active_sub_time-6)>=0)
{
$active_sub_time-=6;
$result+=($arr_subscription_values['COST_6']);
}
while(($active_sub_time-3)>=0)
{
$active_sub_time-=3;
$result+=($arr_subscription_values['COST_3']);
}
while(($active_sub_time-1)>=0)
{
$active_sub_time-=1;
$result+=($arr_subscription_values['COST_1']);
}
if($active_sub_time>0)
$result+=($active_sub_time)*($arr_subscription_values['COST_1']);
return $result;
}
$datetime1 = date_create('2009-12-11');
$datetime2 = date_create('2010-11-09');
$interval = date_diff($datetime1, $datetime2);
$num_of_months = ($interval->format('%y'))*12+($interval->format('%m'))+($interval->format('%a'))/30;
echo "<br />";
$v = array('COST_1'=>'3.99','COST_3'=>'9.99','COST_6'=>'15.99','COST_12'=>'25.99');
echo "OPT value for $num_of_months months=" . optimized_subscription_total($num_of_months, $v);
?>
Strangely I get the bug appearing only after 7 to 10 times after refreshing this code.
So I got:
OPT value for 10 months=M.97
as a result here. I think I need to get a float number, no ?
I was expecting the result of the function that should be "OPT value for 10 months=29.97", as it should take COST_6 + COST_3 + COST_1... but I get that weird M.97, and sometimes things like POKHHHG.97

Answer 1

I would change the logic to the following and see if error is still produced. I think this is a little bit more clear and easy to debug. It is the same as your though just explained differently.
while($active_sub_time>=12)
{
$active_sub_time-=12;
$result+=($arr_subscription_values['COST_12']);
}
while($active_sub_time>=6)
{
$active_sub_time-=6;
$result+=($arr_subscription_values['COST_6']);
}
while($active_sub_time>=3)
{
$active_sub_time-=3;
$result+=($arr_subscription_values['COST_3']);
}
while($active_sub_time>=1)
{
$active_sub_time-=1;
$result+=($arr_subscription_values['COST_1']);
}
What I would also do is added debug cod at the top inside function.
echo "<br>$active_sub_time" // debug code include at the top
This will give you an idea, if any garbage being is being sent to the function itself.
Also I would add this test function in all while blocks if the above does not resolve the issue.
if (!is_numeric($result))
{
echo"<br> Bug occurred";break; // print other values if necessary
}