Format money, calculated by multiplying hours * rate (multiple entries in db) - php

Question

I am facing a difficult problem here, and i don't know how to solve it.
I have 2 different tables. A Tasks table and Projects table. The tasks table holds time spent(in seconds), the rate per hour for that specific task, and in which project does that entry belong to. So basically something like this:
{id} - {seconds} - {rate} - {projectId}
Now, i am calculating the billing rate per hour for a specific task like this:
function calc ($Totalseconds, $rate){
$minutes = $Totalseconds/60; //minutes
$money = $rate * ($minutes / 60); //hours
return $money;
}
...
$money = calc($task['seconds'], $task['rate']);
$value = number_format($money,2); //ex. 1,824.69
This of course works well....For example:
task1 is 1,824.69
task2 is 24.33
etc....
As long as a project has only a few tasks, then the total billing rate for that project is correct. But when a project has more than a few tasks under it, then the total value is a bit different than when i am adding them all manually with a calculator...So in another page i am showing the total billing rate for a specific project by calculating all tasks and rates for that specific projec.
I am 'selecting' all tasks from the db for a specific projectId in an array, then do:
//array projArray has the sum of all seconds and rate for every task under a specific project
//for example a project has 10 tasks with rate $12 p/h and 2 task with $10 p/h
//so projArray has 2 arrays. One for the total time in seconds for the 10 tasks + their rate and one for the 2 tasks + their rate.
$total = 0;
foreach($projArray as $value) {
$total += calc($value['TotalTime'], $value['rate']);
}
$Totalvalue = number_format($total,2);
And here is the problem...When i am manually adding all the task's billing rate, for a specific project, with a calculator i get 2,426.91 but this last function returns 2,426.92 instead.
The seconds and rates are correct in both cases. I dont get it why is there a difference with the final value ?
I guess the more tasks i have under a project the bigger the difference ? Right now i am testing things with 1-50 tasks...for each project

Answer 1

Example data:
a: 1,233
b: 1,233
c: 1,233
number_format((a + b + c), 2) = 3,70
number_format(a, 2) + number_format(b, 2) + number_format(c, 2) = 3,69
When you call number_format on each task individually, 0,003 is being rounded off.
Something similar is probably happening in your calculations.