phpseclib\Crypt\Common\PKCS1::generateSymmetricKey PHP Method

    static function generateSymmetricKey($password, $iv, $length)
    {
        $symkey = '';
        $iv = substr($iv, 0, 8);
        while (strlen($symkey) < $length) {
            $symkey .= md5($symkey . $password . $iv, true);
        }
        return substr($symkey, 0, $length);
    }