Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/TextData.php
public static function PROPERCASE($mixedCaseString)
    {
        $mixedCaseString = Functions::flattenSingleValue($mixedCaseString);

        if (is_bool($mixedCaseString)) {
            $mixedCaseString = ($mixedCaseString) ? Calculation::getTRUE() : Calculation::getFALSE();
        }

        return StringHelper::strToTitle($mixedCaseString);
    }
public static function PROPERCASE($mixedCaseString)
    {
        $mixedCaseString = Functions::flattenSingleValue($mixedCaseString);

        if (is_bool($mixedCaseString)) {
            $mixedCaseString = ($mixedCaseString) ? Calculation::getTRUE() : Calculation::getFALSE();
        }

        return StringHelper::strToTitle($mixedCaseString);
    }
PROPERCASE.

Converts a string value to upper case.

@param string $mixedCaseString

@return string