note 26842 deleted from language.operators.arithmetic by sniper

Note Submitter: tld@tld.digitalcurse.com ---- Be aware of the % operator: Modulus(A,B) is defined as the remainder of the division of A by B for A and B natural (positive integers). The extension to integer A (-1, -2, -3 and so on) can be done in two different ways: the first is by congruence (A1 is congruent to A2 mod B if A1 + k*B = A2 for an integer k), the other is by absolute value (for a negative A, A mod B = -((-A) mod B) ). Values returned are in range [0, B) in the first case and (-B, B) in the second. The first method validates (in PHP) [floor(A / B) * B + A % B == A], while the second validates [sign(A)*floor(abs(A/B))* B + A % B == A] BE AWARE! PHP implements the second method! This is not considered a bug by PHP coders, yet it can cause a number of unexpected problems with mathematical axioms: if ((-1 % 3) != (2 % 3)) { echo 'They are different'; } WILL get to the echo, even if -1 = 2 mod 3; operations on time (no matter if 24h or 12h) might end up with negative hours and so on. To have the congruence method you can use this function: function mod($a, $b) { return ((($a % $b) + $b) % $b); } and then you'll have (mod(-1, 3) == mod(2, 3)), as expected. :)