PHP calculation logic issue - php

Question

Not sure if I'm being stupid here. Probably something obvious but when you've been staring at the same issue for hours on end it starts to drive you crazy.
I'm doing a few calcuations using PHP, all fairly straight forward.
I have a table called sales, say:
total, costs
424.53, 125
853.91, 125
To get the data I need...
gross = total - cost
vat = gross - ( gross / 1.2 )
profit = gross - vat
I need to generate a report, so for each row in the sales database I need to loop over and run the above calculations to get the data I need.
If I add the sum of total and the sum of costs, and then work out the gross, vat and profit above, and round vat and profit to 2 decimal plates the values are as expected.
The problem I'm having is where I'm looping over each row and calculating gross, vat and profit. If I don't round vat and profit on each row, but round the final totals, they match the values where I add sum(total) and sum(costs).
But then in the report I generate, if I don't round vat and profit then they don't show to two decimal places, which I need.
Actual code is below, pretty sure it's more of a logic issue than code.
$sum = 0; // Test variable
foreach( .. as ... )
{
// Assigning $total and $cost
$gross = $total - $cost;
$data['profit'] = $gross;
// If I round this VAT so vat shows to two decimal points, $sum becomes off by some pence.
// If I don't round it but then round $sum after the loop, it matches the echo statement value which is the correct amount
$vat = $this->vat( $gross );
$data['vat'] = $vat;
$profit = $gross - $vat;
$data['net_profit'] = $profit;
$sum += $profit;
$array[] = $data;
}
echo "131547.82<br><br>";
echo $sum;
die;

Answer 1

It's an accuracy problem caused by using floats.
When you do calculations with pure PHP you need to be careful.
You may run into glitches, when comparing two floats.
I would suggest to use some helper function or a currency / money object in order to work with them. It might be better to use a PHP Extension for math stuff, like the PHP Extensions BCMath, which has for instance the function bcadd(). Anyway, here are some helpers, which you might use in your calculation loop.
/**
* turn "string float" into "rounded float with precision 2"
* (string) 123,19124124 = (float) 123.19
*
* #param type $string
*/
function formatNumber($string, $precision = 2)
{
return round((float) str_replace(',', '.', $string), $precision);
}
There is also sprintf: echo sprintf("%.2f", $a);.
These two are based on PHP's NumberFormatter from the Intl Extension.
// 123,19 EUR = 123.19
function parseNumber($string_number)
{
$fmt = numfmt_create('de_DE', \NumberFormatter::DECIMAL);
return numfmt_parse($fmt, $string_number);
}
// 123.19 = 123,19 EUR
function numberFormat($value)
{
$f = \NumberFormatter::create("de_DE", \NumberFormatter::CURRENCY);
return $f->formatCurrency($value, 'EUR');
}
For comparing two floats:
/**
* Do not check, that the numbers are exactly the same,
* but check that their difference is very small!
* A really small rounding error margin (epsilon) is expected.
* It's an equals check within defined precision.
*/
function compareFloats($a, $b)
{
return (abs(($a - $b) / $b) < 0.000001) ? true : false;
}