PHP random string generation function

function random_string($length, $max=FALSE)
{
 if (is_int($max) && $max > $length)
  {
    $length = mt_rand($length, $max);
  }
  $output = '';

  for ($i=0; $i<$length; $i++)
  {
    $which = mt_rand(0,2);

    if ($which === 0)
    {
      $output .= mt_rand(0,9);
    }
    elseif ($which === 1)
    {
      $output .= chr(mt_rand(65,90));
    }
    else
    {
      $output .= chr(mt_rand(97,122));
    }
  }
  return $output;
}