How is -13 d = -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

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.