php - Generating unique 6 digit code -

i'm generating 6 digit code following characters. these used stamp on stickers.
generated in batches of 10k or less (before printing) , don't envisage there ever more 1-2 million total (probably less).
after generate batches of codes, i'll check mysql database of existing codes ensure there no duplicates.

// exclude problem chars: b8g6i1l0oqds5z2  $characters = 'acefhjkmnprtuvwxy4937';  $string = '';  ($i = 0; $i < 6; $i++) {     $string .= $characters[rand(0, strlen($characters) - 1)]; }     return $string; 

1. is solid approach generating code?
2. how many possible permutations there be? (6 digit code pool of 21 characters). sorry math isn't strong point

21^6 = 85766121 possibilities.

using db , storing used values bad. if want fake randomness can use following:

reduce 19 possible numbers , make use of fact groups of order p^k p odd prime cyclic.

take group of order 7^19, using generator co-prime 7^19 (i'll pick 13^11, can choose not divisible 7).

then following works:

$previous = 0;  function generator($previous) {    $generator = pow(13,11);   $modulus = pow(7,19); //int might small   $possiblechars = "acefhjkmnprtuvwxy49";    $previous = ($previous + $generator) % $modulus;   $output='';   $temp = $previous;    for($i = 0; $i < 6; $i++) {     $output += $possiblechars[$temp % 19];     $temp = $temp / 19;   }    return $output; } 

it cycle through possible values , little random unless go digging. safer alternative multiplicative groups forget math :(