code: public static function STRINGLENGTH($value = '')
      {
          $value = Functions::flattenSingleValue($value);
  
          if (is_bool($value)) {
              $value = ($value) ? Calculation::getTRUE() : Calculation::getFALSE();
          }
  
          return mb_strlen($value, 'UTF-8');
      }
docs: STRINGLENGTH.
  
  @param string $value Value
  
  @return int