Make sure exactly one boolean of a given list is true?

Question

If I have the following booleans

const YESTERDAY = false;
const TODAY = true;
const TOMORROW = false;

What code can I write to make sure exactly one is true?

I've tried this:

$x = self::YESTERDAY ^ self::TODAY ^ self::TOMORROW;

The problem is that with all three constants set to true then $x is true.

Answer 1

$x = ((int) self::YESTERDAY) + ((int) self::TODAY) + ((int) self::TOMORROW); Then if $x === 1; You've got what you need.

EDITED:

Even without type casts (int), it works well, thanks to @DaveRandom, so:

if (self::YESTERDAY + self::TODAY + self::TOMORROW == 1) {}, as for me.

Answer 2

The neatest way I can think of is array_sum():

if (array_sum(array(self::YESTERDAY, self::TODAY, self::TOMORROW)) == 1) {
  // Do something
}

EDIT Actually, all you need to do it replace the ^ with + in your original attempt, and it achieves the same thing:

$x = self::YESTERDAY + self::TODAY + self::TOMORROW;

This turns $x into the number of TRUE values. So for a boolean output use:

$ok = self::YESTERDAY + self::TODAY + self::TOMORROW === 1;