PHP, generate string with format, check against SQL db? -

i'm trying use php way process in webpage, typical language java unfamiliar how done product keys.

this process:

1. generate random string format xx-xxxx-xxxx-xxx mix of numbers , letters. 2. check if exists in sql database 3. if exists, generate 1 , repeat? 

so how using php? please explain need , best way of going it.

generate random string following function.

<?php function randomstring() {     $alphabet = "abcdefghijklmnopqrstuwxyzabcdefghijklmnopqrstuwxyz0123456789";     $pass = array(); //remember declare $pass array     $alphalength = strlen($alphabet) - 1; //put length -1 in cache     $array_lengths = array(2,4,4,3);     foreach($array_lengths $v){       ($i = 0; $i < $v; $i++) {         $n = rand(0, $alphalength);         $pass[] = $alphabet[$n];       }       $pass[] = '-';     }     return rtrim(implode($pass),'-'); //turn array string }  echo randomstring();  ?> 

sql please create unique key field , use on duplicate key query insert/ update data

demo