Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/TextData.php
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 $returnValue;
    }
public static function CONCATENATE(...$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;
    }
CONCATENATE.

@return string