Extending The Controller Class in CodeIgniter

I take it you have put your MY_Controller in /application/core, and set the prefix in the config. I would be careful about using index as a class name though. As a function/method in Codeigniter it has a dedicated behaviour.

If you then want to extend that controller you need to put the classes in the same file.

E.g. In /application core

/* start of php file */
class MY_Controller extends CI_Controller {
    public function __construct() {
       parent::__construct();
    }
...
}

class another_controller extends MY_Controller {
    public function __construct() {
       parent::__construct();
    }
...
}
/* end of php file */

In /application/controllers

class foo extends MY_Controller {
    public function __construct() {
       parent::__construct();
    }
...
}

or

class bar extends another_controller {
    public function __construct() {
       parent::__construct();
    }
...
}

better to pass the value in the hidden field

<form class="addinvestmentform" action="<?php echo base_url();?>index.php/ctl_dbcont/input_investment" name="application" method="post" >
<input type="hidden" name="my_id" value="<?php echo $id; ?>"/>
</form>

in your ci function

function input_investment() {
    $id = $this->input->post('my_id');
    $this->load->helper('form');
    $this->load->helper('html');
    $this->load->model('mod_user');
    $this->mod_user->insertinvestment($id);
}

or if you want (A test)

// Sample view

<?php $id = 1; ?>
<form action="<?php echo base_url('my_class/my_method/' . $id); ?>" method="post" >
  <input type="submit" />
</form>

// Controller

class My_class extends CI_Controller {

  public function index() {
    $this->load->view('my_class');
  }

  public function my_method($id) {
    echo $id; // outputs 1
  }

}

I think the problem is with short tags,

<?php include 'header.php';?>

But i would say, in codeigniter use,

$this->load->view("header.php");