code: public static function getMutation(array &$tokens, $index)
      {
          $num = (float) $tokens[$index][1];
          if ($num == 0) {
              $replace = 1.0;
          } elseif ($num == 1) {
              $replace = 0.0;
          } elseif ($num < 2) {
              $replace = $num + 1;
          } else {
              $replace = 1.0;
          }
          $tokens[$index] = [
              T_DNUMBER,
              sprintf("%.2f", $replace)
          ];
      }
docs: Replace 0.0 with 1.0, 1.0 with 0.0, and float between 1 and 2 is incremented
  by one, and any float greater than 2 is replaced with 1.0.
  
  @param array $tokens
  @param int $index
  @return array