Codeigniter使用config.php将非www重定向到www

I know how to redirect non-www to www using the .htaccess. But for some reason, I can't do it from the .htaccess file! I would like to do this from application/config.php. There already have HTTP to https redirect. here is the code,

$root = (isset($_SERVER['HTTPS']) ? "https://" : "http://") . $_SERVER['HTTP_HOST'];
$root .= str_replace(basename($_SERVER['SCRIPT_NAME']), '', $_SERVER['SCRIPT_NAME']);
$config['base_url'] = $root;

Now, Is it possible to redirect non-www to www from here? If it's possible then how?

Thanks in advance.

Ok, after trying in several ways I found a simple solution! Unfortunately, I couldn't do this from the config.php. But I did this from the index.php without using .htaccess. And for me, it's working well.

Here is the code, that I implemented at root-> index.php

if (substr($_SERVER['SERVER_NAME'],0,3)!="www"){
    header("HTTP/1.1 301 Moved Permanently");
    header("Location: https://www.".$_SERVER['HTTP_HOST'] ."".$_SERVER['REQUEST_URI']);
    exit(3); // EXIT_CONFIG
}