Generate save activation key as product key

I am trying to create a function which creates a random String. This String should consist of letters (only caps) and numbers. It will be used to activate a product. So the user has to type it into a text field. So far I found the following function:

function random_str($length, $keyspace =  '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'){

  $pieces = [];
  $max = mb_strlen($keyspace, '8bit') - 1;
  for ($i = 0; $i < $length; ++$i) {
      $pieces []= $keyspace[random_int(0, $max)];
  }
  return implode('', $pieces);
}

I do not have that much experience with random functions. So is there anything to improve or would you suggest a different function?