Can't explain php modulo result

Question

Can someone please explain what is going on here?

-1 % 7 = 6 https://www.wolframalpha.com/input/?i=-1%257 ...but

echo (-1 % 7)."\n";    // prints -1 WRONG
echo fmod(-1,7)."\n";  // prints -1 WRONG

I also tried these examples from php.net, all of them return correctly.

echo (5 % 3)."\n";           // prints 2
echo (5 % -3)."\n";          // prints 2
echo (-5 % 3)."\n";          // prints -2
echo (-5 % -3)."\n";         // prints -2

PHP is giving me a result I can't explain for -1%7. It's not overflowing the int but I tried fmod anyways, but I'm still having my same issue (ie Doesn't look to be this issue: php modulo return wrong result)

Answer 1

So, turns out -1 is a correct answer. It's the "negative remainder". I'm not a math major, I wasn't aware there was another valid-ish answer. I'm looking exclusively for the non-negative remainder (the normal answer).

Turns out I can use gmp_mod for that. https://www.php.net/manual/en/function.gmp-mod.php

This helped me get on the right track: https://www.omnicalculator.com/math/modulo

Answer 2

If you want to get the true modulo of a negative number in PHP, you have to do two remainder operations, the second after adding the number you are doing the modulo by to make the input to the operation positive:

$a = ((-1 % 7) + 7) % 7;
echo $a;
$a = ((4 % 7) + 7) % 7;
echo $a;

Output

6
4

Demo on 3v4l.org