code: public static function CONCATENATE(...$args)
      {
          $returnValue = '';
  
          // Loop through arguments
          $aArgs = Functions::flattenArray($args);
          foreach ($aArgs as $arg) {
              if (is_bool($arg)) {
                  if (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_OPENOFFICE) {
                      $arg = (int) $arg;
                  } else {
                      $arg = ($arg) ? Calculation::getTRUE() : Calculation::getFALSE();
                  }
              }
              $returnValue .= $arg;
          }
  
          return $returnValue;
      }
docs: CONCATENATE.
  
  @return string