问将自动递增ID转换为9位随机序列号

EN

serial = id^base mod p
serial = 42^17 % 27407 = 24978

id = serial^inv mod p
id = 24978^12897 % 27407 = 42

$secretKey = 'oh no nobody will guess this';
$userId = 312;
$serialNumber = hash('sha256', $secretKey . $userId);

$hash = sha1("secretanswer".$userid);
$trimmedhash = substr($has,0,9); //get a 9 digit random hash

   $userid = "getthissomehow";
   $hash = sha1("secretanswer".$userid);
   $trimmedhash = substr($has,0,9);

   $prevhash = "asfaf23rfs"; //get previously stored hash somehow
   if($trimmedhash == $prevhash) { 
     //valid
   }