code: public static function MID($value = '', $start = 1, $chars = null)
      {
          $value = Functions::flattenSingleValue($value);
          $start = Functions::flattenSingleValue($start);
          $chars = Functions::flattenSingleValue($chars);
  
          if (($start < 1) || ($chars < 0)) {
              return Functions::VALUE();
          }
  
          if (is_bool($value)) {
              $value = ($value) ? Calculation::getTRUE() : Calculation::getFALSE();
          }
  
          if (empty($chars)) {
              return '';
          }
  
          return mb_substr($value, --$start, $chars, 'UTF-8');
      }
docs: MID.
  
  @param string $value Value
  @param int $start Start character
  @param int $chars Number of characters
  
  @return string