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需要安装openssl扩展，openssl_random_pseudo_bytes

function getRandomString($length = 6)
{
        /*
         * Use OpenSSL (if available)
         */
        if (function_exists('openssl_random_pseudo_bytes')) {
            $bytes = openssl_random_pseudo_bytes($length * 2);

            if ($bytes === false)
                throw new RuntimeException('Unable to generate a random string');

            return substr(str_replace(['/', '+', '='], '', base64_encode($bytes)), 0, $length);
        }

        $pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

        return substr(str_shuffle(str_repeat($pool, 5)), 0, $length);
}