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Generate unique 4 char values

Posted on 2007-03-28
Medium Priority
Last Modified: 2008-02-01
I need to generate some PINs for a login system. I only have 39 values but I would like them to be unique,
how would I do that in PHP?
Question by:EddieShipman
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Expert Comment

ID: 18808895
There are several ways.

Are you saving the pins somewhere to make sure that they are not reused?
If so - we could do something like this:

$uniqueval = rand(0,9999);
/// do some code to get a list of previously assigned pins (not shown)
// (now do a while loop to go through the previously assigned vals)
while($x = 0)



Expert Comment

ID: 18808921
oops hit a button on the keyboard and it submitted - try again

if(in_array($uniqueval,$something) {   $uniqueval=rand(0,9999); $x =0; }
else { $x = 1; }

At the conclusion of the loop $uniqueval should be a unique 4 digit value not previously assigned.
LVL 26

Author Comment

ID: 18809051
I have 39 records in a mySQL table named Teams, the field is PIN.
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LVL 29

Accepted Solution

rdivilbiss earned 2000 total points
ID: 18809082
If you want a 4 digit pin, then rand(0,9999); needs to be rand(1111,9999);

$pins = array();
$maxPins = 39;

for ($idx=0; $idx<$maxPins; $idx++) {
      $unique = false;
      while (!$unique) {
            $tmpPin = rand(1111,9999);
            if (!array_search($tmpPin, $pins)) {
       // you can update a table row $idx here

LVL 27

Expert Comment

by:Cornelia Yoder
ID: 18809085
                $result=mysql_query("Select * from Teams where PIN=$newpin;");

At the end of this loop, you will have $newpin equal to a random value that is not in the table.
LVL 29

Expert Comment

ID: 18809207
Why do multiple database lookups, when you can determine the uniqueness in code alone, and only hit the database when you update a record with the new PIN.

Also, (although not explicitly stated by the asker,) a PIN is normally a 4 digit integer, where rand()%1000000000; will return many different lengths.
LVL 27

Expert Comment

by:Cornelia Yoder
ID: 18809271
OK, so just change the rand statement to generate whatever size number you want for the pin.

However, unless the pins are all generated at the same time and only once, you cannot save them in code.  You will have to do database lookups each time you generate a new one for a new user.
LVL 29

Expert Comment

ID: 18809329
He says he already has 39 rows, so only hit each row one time, e.g. to update it with the unique PIN.

If he was adding a new row subsequent, then your query until unique would be a good choice.

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