PHP modulo operator (%) returns negative numbers

The PHP modulo operator, %, returns negative numbers. e.g. -3%7 == -3. Other languages will wrap the negative to produce a positive number, e.g. -3%7 == 4.

To implement a true modulo operator, you have to write a custom function:

function truemod($num, $mod) {
  return ($mod + ($num % $mod)) % $mod;
}

Example: trumod(-3,7) == 4

References

• Modulo operation - Wikipedia
• PHP Bugs: #20365: the modulo operator % doesn't work with negative integers!
• AtariAge Forums -> In Search of the True Modulus (an epic)