CCNA, CCNP, MCSA, CCNA Final Exam, All Answer Test Module With 100/100

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 .