Advanced Algoithm issues to generate unique 6 char long code - php

Question

I have used this algorithm to generate 100 codes. Now Once again i have searched the threads on this site and on google before posting. The algorthms found do work in generating the code the issue I got is i have to generate UNIQUE codes from the one i already have. The problem i am finding is all the scripts are all failing to execute.
So basically I have an ARRAY holding 100 voucher codes I need to generate 400 more voucher codes however while my algorithm's works it keeps timing out due to it taking to long to execute. I have tried several other algorithms from the threads on Stack overflow but for some reason they all keep timing out. I need advice on what I can do to my algorithm to actually produce 400 more unique 6 Char codes.
Here is script I have
/**
**Use this portion to get my current 100 codes in an array which i use
**/
$redeem = 'THIS IS A STRING WHICH CREATES AN ARRAY CONTIANING 100 VALUES';
$redeem = preg_replace('/\s+/', '', $redeem);
$redeem_code = str_split($redeem, 6);
/**
**Function generates unique 6 letter voucher codes
**
**/
function random_str($length, $keyspace = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ')
{
$str = '';
$max = mb_strlen($keyspace, '8bit') - 1;
for ($i = 0; $i < $length; ++$i) {
$str .= $keyspace[mt_rand(0, $max)];
}
return $str;
}
/**
**This is the query which checks against my array and then echo's a valid code
**
**/
$redeem_count = count($redeem_code);
$i = 0;
$ia = 1;
while($i <= 500){
$string = random_str(6);
if(in_array($string, $redeem_code))
{
echo $string;
$i= $i+1;
}
}
Constant Error Message on All functions tried.
Fatal error: Maximum execution time of 120 seconds exceeded in C:\wamp\www\nfr_chip.php on line 115

Answer 1

You never add anything to the array and only increase $i if the code already exist.
This should work...
$redeem_count = count($redeem_code);
while($redeem_count <= 500) {
$string = random_str(6);
if(!in_array($string, $redeem_code))
{
$redeem_code[] = $string;
$redeem_count++;
}
}
And for the way you are splitting your string with codes, instead of using preg_replace (avoid regex unless you really need it):
$redeem_code = explode(" ", $redeem);

Answer 2

You can safely use openssl_random_pseudo_bytes with bin2hex to generate pseudo-random, as the name implies, strings.
Here's an example:
for ($i=1;$i<=5;$i++){
print bin2hex(openssl_random_pseudo_bytes(3)) . PHP_EOL;
}
//output
9282fb
3b9798
c187a0
a058e3
df2e4b
3 bytes will give you a 6 char string