code: public static function getMutation(array &$tokens, $index)
      {
          $num = (integer) $tokens[$index][1];
          if ($num == 0) {
              $replace = 1;
          } elseif ($num == 1) {
              $replace = 0;
          } else {
              $replace = $num + 1;
          }
          $tokens[$index] = [
              T_LNUMBER,
              (string) $replace
          ];
      }
docs: Replace 1 with 0, 0 with 1, or increment.
  
  @param array $tokens
  @param int $index
  @return array