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

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

        return mb_strlen($value, 'UTF-8');
    }
public static function STRINGLENGTH($value = '')
    {
        $value = Functions::flattenSingleValue($value);

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

        return mb_strlen($value, 'UTF-8');
    }
STRINGLENGTH.

@param string $value Value

@return int