I used the following.
My goal is to use CodeIgniter for some pages and leave the rest of the pages in Wordpress intact. I have only a few steps for this:
Copy the CI folder in the root directory of Wordpress
Modify the "index.php" CI to add include, which will add WP functions to CodeIgniter:
@require '../wp-load.php'; require_once BASEPATH.'core/CodeIgniter.php';
After adding this line, Wordpress functions will be used in CodeIgniter; they will be used primarily for viewing.
Modify .htaccess WP to not rewrite CI URLs: After this line:
RewriteRule ^index\.php$ - [L]
Add this line:
RewriteCond %{REQUEST_URI} !^/(codeigniter_folder|codeigniter_folder/.*)$
CI views can then use WP features.
source share