Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Shared/StringHelper.php
public static function sanitizeUTF8($value)
    {
        if (self::getIsIconvEnabled()) {
            $value = @iconv('UTF-8', 'UTF-8', $value);

            return $value;
        }

        $value = mb_convert_encoding($value, 'UTF-8', 'UTF-8');

        return $value;
    }
public static function sanitizeUTF8($value)
    {
        if (self::getIsIconvEnabled()) {
            $value = @iconv('UTF-8', 'UTF-8', $value);

            return $value;
        }

        $value = mb_convert_encoding($value, 'UTF-8', 'UTF-8');

        return $value;
    }
Try to sanitize UTF8, stripping invalid byte sequences. Not perfect. Does not surrogate characters.

@param string $value

@return string