How is -13 % 64 = -13 in PHP?

Question

Derived from this question : (Java) How does java do modulus calculations with negative numbers?

Anywhere to force PHP to return positive 51?

update
Looking for a configuration setting to fix, instead hard-guessing

Or other math function like bcmath?

updated
Not entire convinced by that java answer, as it does not take account of negative modulus -13+(-64) =?

Answer 1

Anyway, the post you referenced already gave the correct answer:

$r = $x % $n;
if ($r < 0)
{
    $r += abs($n);
}

Where $x = -13 and $n = 64.

Answer 2

If GMP is available, you can use gmp_mod

Calculates n modulo d. The result is always non-negative, the sign of d is ignored.

Example:

echo gmp_strval(gmp_mod('-13', '64')); // 51

Note that n and d have to be GMP number resources or numeric strings. Anything else won't work¹

echo gmp_strval(gmp_mod(-13, 64));
echo gmp_mod(-13, 64);

will both return -51 instead (which is a bug).

¹ ^{running the above in this codepad, will produce 51 in all three cases. It won't do that on my development machine.}

Answer 3

The modulo operation should find the remainder of division of a number by another. But strictly speaking in most mainstream programming languages the modulo operation malfunctions if dividend or/and divisor are negative. This includes PHP, Perl, Python, Java, C, C++, etc.

Why I say malfunction? Because according to mathematic definition, a remainder must be zero or positive.

The simple solution is to handle the case yourself:

if r < 0  then r = r + |divisor|;

|divisor| is the absolute value of divisor.

Another solution is to use a library (as @Gordon pointed). However I wouldn't use a library to handle a simple case like this.