Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/StringHelper.php
public static function strCaseReverse($pValue)
    {
        $characters = self::mbStrSplit($pValue);
        foreach ($characters as &$character) {
            if (self::mbIsUpper($character)) {
                $character = mb_strtolower($character, 'UTF-8');
            } else {
                $character = mb_strtoupper($character, 'UTF-8');
            }
        }

        return implode('', $characters);
    }
public static function strCaseReverse($pValue)
    {
        $characters = self::mbStrSplit($pValue);
        foreach ($characters as &$character) {
            if (self::mbIsUpper($character)) {
                $character = mb_strtolower($character, 'UTF-8');
            } else {
                $character = mb_strtoupper($character, 'UTF-8');
            }
        }

        return implode('', $characters);
    }
Reverse the case of a string, so that all uppercase characters become lowercase
and all lowercase characters become uppercase.

@param string $pValue UTF-8 encoded string

@return string