Core_Captcha::random

protected static function random($length = 8)
{
    $pool = '2345679ACDEFHJKLMNPRSTUVWXYZ';

    $str = '';

    $pool_size = strlen($pool);

    for( $i = 0; $i < $length; $i ++ )
    {
        $str .= substr($pool, mt_rand(0, $pool_size - 1), 1);
    }

    return $str;
}