<PHP> how to generate a unique field without auto-increment - php

Question

i'm trying to generate a unique id using this code:
$RCode = md5(uniqid(rand(), true));
then i want it to check in my database if the RCode is unique.
if it isnt i want it to use that part of the code again and check in my database again if it is unique,
if it is unique it should write to my database.
i have all the code i need for checking and writing into the database, i just have no idea how to make it loop back to the start.
Help is appreciated!
Thanks a lot in advance!

Answer 1

Don't bother checking at first. Instead put a unique constraint on the column, that way the insert will fail if the RCode isn't unique. Then you can handle that error/exception and try another hash. The probability of a collision is low so in this case you probably aren't going to be hammering the database.

Answer 2

Typical example for do-while loop.
Some PHP-pseudocode:
do {
$rcode = md5(uniqid(rand(), true));
$res = mysql_result(mysql_query("SELECT COUNT(*) FROM records WHERE rcode='$rcode'"));
} while ($res[0] > 0);
mysql_query("INSERT INTO records (rcode) VALUES ('$rcode')");

Answer 3

$found = false;
while (! $found) {
//try..
if (...unique...) {
$found = true;
}
}

Answer 4

Going to the start is as easy as implementing a while loop. Heck, you could even use goto (kidding!).
But I don't understand why you don't want to use auto_increment.

Answer 5

You can use the "loop forever then break out on success" method:
while (true) {
$RCode = ...;
if ($RCode does not exist in db) {
break;
}
}
write to the db
Edit: Or better, make sure the field has a unique constraint on it, then test for uniqueness by checking for failure of an insert:
while (true) {
$RCode = ...
try to insert RCode
if (no failure) {
break;
}
}
This will be more resilient to concurrent hits.

Answer 6

This answer is a little different to what you asked but it solves the problem of a unique ID in a different way that may be better to use depending on your application.
To be honest I like to use variables such as date and time combined with another variable such as an IP address for this type of thing, you can then be very certain that your ID will be unique because the date and time will not reoccur and in the event there are 2 requests in the same second the IP address of the user is completely unique at this time also. Plus no having to check with the database. An example would be
$idstring = date('Ymdhis');
$ipstring = $_SERVER['REMOTE_HOST'];
$hashme = $idstring.$ipstring;
$idhash = md5($hashme);
I hope this is helpful.

Answer 7

As you hash uniqid() returned value using md5() (the same goes for any other hashing algorithm), possibility of getting not unique string is extremely low.
In my opinion, its so low that checking for that string to be unique would be overkill.
All you need to do is insert value in the database. It will be unique!
Edit:
You should use uniqid(null, true) to get 23 chars long, unique string. If that is what you need - unique string.

Answer 8

you can just run MD5 on the timestamp...
$uniqueid = md5(time());

Answer 9

First, don't md5 uniqueid. uniqueid is good enough and it does not have the overhead of md5 if used by itself.
Second, you are far better off obscuring an automatically incrementing number than using a UUID or some equivalent.
But, if you must:
do {
$rcode = uniqid(rand(), true);
$res = mysql_query("SELECT COUNT(*) FROM records WHERE rcode='$rcode'");
} while ($res && mysql_num_rows($res));
mysql_query("INSERT INTO records (rcode) VALUES ('$rcode')");
// OR!
// assuming unique index on rcode
do {
$rcode = uniqid(rand(), true);
} while (mysql_query("INSERT INTO records (rcode) VALUES ('$rcode')"););