Question

I have this small internal project that inserts occasional entries into a MySQL database. I have a column named “idChar” were I set it’s value to a randomly generated string using 62 possible characters with a length of 31.

Today I discovered that a new entry just so happened to have the same exact idChar as an entry from several months ago. I am now checking for duplicate entries before saving them, but this made me think about the odds of this happening and I am curious to know if my implementation of generating these random keys is flawed. Getting a duplicate should be roughly 1 in 62^31 right?

function getCode($len)
{
    //$len = 10;
    $base='ABCDEFGHIJKLMNOPQRSTWXYZabcdefghijklmnopqrstwxyz123456789';
    $max=strlen($base)-1;
    $linkCode='';
    mt_srand((double)microtime()*1000000);
    while (strlen($linkCode)<$len+1)
      $linkCode.=$base{mt_rand(0,$max)};

    return $linkCode;
}

$idChar=getCode(30);

//code to insert into MySQL here

Answer 1

The odds of getting a duplicate would be calculated as per the birthday problem, because that’s how you calculate the chance of collisions for the output of a function that produces a randomly chosen output from a discrete codomain. In effect you want to calculate the chance that among a pool of selections made randomly any two selections are the same.

You should also completely drop the mt_srand call as it is not necessary, and it’s likely to provide worse seeds than what PHP will do automatically. Consider that the output of microtime (in my system at least) is like

0.29574400 1348356024

which means that you only have 1 million different seeds available as the last two digits of the float are always zeroes and the (double)microtime() cast completely ignores the seconds part (it would be a lousy seed anyway).

Assuming that the random number generator produces the same sequence of random numbers whenever it is seeded with the same seed then in effect you only have 1 million possible random codes instead of 62^31 — quite a decrease! Fortunately it is documented that this does not happen on PHP 5.2.1 onwards.