Is PHP's `precision` Workaround Reliable for Precise 2-Digit Money Calculations?

Can I Rely on PHP PHP.INI Precision Workaround for Floating Point Issue?

Introduction

Floating-point arithmetic involves representing fractional numbers as binary fractions, which may not have exact representations. This can lead to rounding errors.

Question 1:

Can this workaround ensure precise 2-digit calculations for money?

Answer 1:

No. PHP precision settings cannot guarantee precise 2-digit calculations, as precision can be increased during calculations.

Question 2:

Example of when the workaround fails:

ini_set('precision', 8);
$a = 5.88; // cost of 1kg
$q = 2.49; // User buys 2.49 kg
$b = $a * 0.01; // 10% Discount only on first kg
echo ($a * $q) - $b; // result: 14.5824 (not precise for money calculations)

Question 3:

Which php.ini.precision value suits best for 2-digit money calculations?

Answer 3:

PHP precision is not a recommended metric for precise financial calculations.

Simple Test

Using bcmath, number_format, and subtraction:

$a = 342349.23;
$b = 341765.07;

ini_set('precision', 20);
echo $a - $b, PHP_EOL; // 584.15999999997438863
echo floatval(round($a - $b, 2)), PHP_EOL; // 584.15999999999996817 (rounding issue)
echo number_format($a - $b, 2), PHP_EOL; // 584.16
echo bcsub($a, $b, 2), PHP_EOL; // 584.15 (precision set to 20)

ini_set('precision', 14);
echo $a - $b, PHP_EOL; // 584.15999999997
echo floatval(round($a - $b, 2)), PHP_EOL; // 584.16
echo number_format($a - $b, 2), PHP_EOL; // 584.16
echo bcsub($a, $b, 2), PHP_EOL; // 584.16 (precision set to 14)

ini_set('precision', 6);
echo $a - $b, PHP_EOL; // 584.16
echo floatval(round($a - $b, 2)), PHP_EOL; // 584.16
echo number_format($a - $b, 2), PHP_EOL; // 584.16
echo bcsub($a, $b, 2), PHP_EOL; // 584.00 (precision set to 6)

ini_set('precision', 3);
echo $a - $b, PHP_EOL; // 584
echo floatval(round($a - $b, 2)), PHP_EOL; // 584
echo number_format($a - $b, 2), PHP_EOL; // 584.16
echo bcsub($a, $b, 2), PHP_EOL; // 0.00 (precision set to 3)

Conclusion

Using number_format for money calculations is recommended for consistency. Storing in cents is also an option.

Update

Question 1: Will precision workaround fail for numbers between 0..999999.99?

Answer 1:

Yes, due to the large number of combinations, a full test is impractical.

Example:

$a = 0.19;
$b = 0.16;
...
$h = $a + $b + ...; // Total value: 0.4

$i = $h - $a - $b - ...; // Result: 1.0408341E-17 (expected: 0.00)

Question 2: How to estimate when precision workaround fails?

Answer 2:

Mathematical solutions exist, but it's not within the scope of this discussion.

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