Multiple Applcation Question

I have to applications using the https://www.codeigniter.com/user_guide/g..._apps.html way

How ever when I type in example.com/admin on the index.php how can I make sure the application folder switches over to admin

$application_folder = 'application/admin';

$application_folder = 'application/catalog';

htdocs > cms     application/admin/
                application/admin/cache/
                application/admin/config/
                application/admin/core/
                application/admin/controllers/
                application/admin/helpers/
                application/admin/hooks/
                application/admin/language/
                application/admin/libraries/
                application/admin/logs/
                application/admin/models/
                application/admin/third_party/
                application/admin/view/
                application/catalog/
                application/catalog/cache/
                application/catalog/config/
                application/catalog/core/
                application/catalog/controllers/
                application/catalog/helpers/
                application/catalog/hooks/
                application/catalog/language/
                application/catalog/libraries/
                application/catalog/logs/
                application/catalog/models/
                application/catalog/third_party/
                application/catalog/view/