public static function randomString($len = null, $chars = null, $cb = null, $pri = 0, $hang = false)
    {
        if ($len === null) {
            $len = 64;
        }
        if ($chars === null) {
            $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-.';
        }
        if ($cb === null) {
            Daemon::log('[CODE WARN] \\PHPDaemon\\Utils\\Crypt::randomString: non-callback way is not secure.'
                . ' Please rewrite your code with callback function in third argument' . PHP_EOL . Debug::backtrace());

            $r = '';
            $m = mb_orig_strlen($chars) - 1;
            for ($i = 0; $i < $len; ++$i) {
                $r .= $chars[mt_rand(0, $m)];
            }
            return $r;
        }
        $charsLen = mb_orig_strlen($chars);
        $mask = static::getMinimalBitMask($charsLen - 1);
        $iterLimit = max($len, $len * 64);
        static::randomInts(2 * $len, function ($ints) use ($cb, $chars, $charsLen, $len, $mask, &$iterLimit) {
            if ($ints === false) {
                $cb(false);
                return;
            }
            $r = '';
            for ($i = 0, $s = sizeof($ints); $i < $s; ++$i) {
                // This is wasteful, but RNGs are fast and doing otherwise adds complexity and bias
                $c = $ints[$i] & $mask;
                // Only use the random number if it is in range, otherwise try another (next iteration)
                if ($c < $charsLen) {
                    $r .= static::stringIdx($chars, $c);
                }
                // Guarantee termination
                if (--$iterLimit <= 0) {
                    return false;
                }
            }
            $d = $len - mb_orig_strlen($r);
            if ($d > 0) {
                static::randomString($d, $chars, function ($r2) use ($r, $cb) {
                    $cb($r . $r2);
                });
                return;
            }
            $cb($r);
        }, $pri, $hang);
    }