I want city1.example.com, city2.example.com, city3.example.com support in CodeIgniter.

Regrettably, $config['base_url'] in config.php sets the bottom URL strictly.

What's the easiest method to discover the framework to aid different subdomains?

You need to most likely just look Codeigniter.com the next time, since it's all there within the wiki:

http://codeigniter.com/wiki/Automatic_base_url/