Setting up one codeigniter for multiple site:
example root folder: /home/myweb/public_html/
to keep it nice and tidy
/home/myweb/ci [all codeigniter files]
/home/myweb/ci/system [codeigniter system folder & sub-folder]
/home/myweb/ci/main [copy of codeigniter /application folder for the main application]
/home/myweb/ci/other [another copy of codeigniter /application folder for the other application]
/home/myweb/public_html/index.php [for the main application]
$system_path = '../ci/system';
$application_folder = '../ci/main';/home/myweb/public_html/subdomain/index.php [for the other application]
$system_path = '../../ci/system';
$application_folder = '../../ci/other';