Generate random unique code - php - php

Question

How do I generate a random code/PIN. I read PHP's rand() and mt_rand() functions and it seems to do good but I found there are duplicates.
Can someone suggest or demonstrate a better way of achieving this in PHP?
my code:
$size = 1000; //handcoded pin-size
function generatePIN($size =500){
$pins = array(); $i = 0; //ini counter
//my basic algorithm for generating unique pin codes
while ($i < $size){
$pin = (mt_rand(100000,999999));
$pins[] = $pin;
++$i;
}
sort($pins);//sort in lowest to highest
return array_unique($pins);//remove duplicates
}
//call it and print or store in database
$pins = generatePIN($size);
foreach ($pins as $pin) {
echo $pin.'<br>';
}
Thank you.

Answer 1

A true random always has the chance to create a duplicate, because it is unconditional to the previous given randoms.
To make a duplicate less likely you can increase the random interval with mt_rand(10000000,99999999).
Or you can check if your number was a duplicate:
while ($i < $size){
$pin = (mt_rand(100000,999999));
if (array_search($pin, $pins) === FALSE) {
$pins[] = $pin;
++$i;
}
}
This will perform more poorly the bigger $size is compared to the random interval and create an endless loop if $size is bigger than it.

Answer 2

adding to the generated value in the loop solved the problem is its very efficient calling it each day. No matter how long the size is.
$size = 1000; //handcoded value
function generatePIN($size =500){
$pins = array(); $i = 0;
//my basic algorithm for generating unique pin codes
while ($i < $size){
$pin = (mt_rand(100000,999999)) + (mt_rand(1000,9999)) + strtotime(date('Y-m-d', strtotime('+'.$i.' week')));
$pins[] = $pin;
++$i;
}
sort($pins);//sort in lowest to highest
return array_unique($pins);//remove duplicates
}
//store in database or print it
$pins = generatePIN($size);
foreach ($pins as $pin) {
echo $pin.'<br>';
}
echo "<hr />";
echo count($pins);