public function getSalt($len)
    {
        $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789`~!@#$%^&*()-=_+';
        $l = strlen($chars) - 1;
        $str = '';
        for ($i = 0; $i < $len; ++$i) {
            $str .= $chars[mt_rand(0, $l)];
        }
        return $str;
    }