CodeIgniter call same function for all controllers

You can develop a helper or library.

codeigniter version 2 : https://codeigniter.com/userguide2/general/creating_libraries.html

codeigniter version 3 : https://codeigniter.com/user_guide/general/creating_libraries.html?highlight=library

You can create MY_Controller class in application/core

Class MY_Controller Extends CI_Controller{

    public function __construct(){
        parent::__construct();
    }

    public function common(){
        // code here
    }
}

Now extend every class with MY_Controller and the function will be available for each class

Class Test Extends MY_Controller{

    public function __construct(){
        parent::__construct();
    }
}

When in the url you call

http://localhost/app/test/common

This will work.

create a model class and create that all function after that extend that model all model classes .you can use one function different controller.other wise you can use Cms helper.this is the best way call a common function in different controller.