如何解决无法修改标头信息 - 已在CodeIgniter中发送的标头

I have read all the answers regarding this error in stackoverflow and couldn't solve it...

There are no whitespaces before and after the opening of <?php tag... and anyway error is showing on line no. 2 ...

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /home/cctvbuzz/public_html/dev/lifetechadmin/application/models/Category_model.php:2)

Filename: helpers/url_helper.php

Line Number: 561

Backtrace:

File: /home/cctvbuzz/public_html/dev/lifetechadmin/application/controllers/Specifications.php
Line: 35
Function: redirect

File: /home/cctvbuzz/public_html/dev/lifetechadmin/index.php
Line: 292
Function: require_once

And here is my Category_model first few line.. where line no. 2 is that class declaration.. So, what else could be the problem.. I also haven't used any function related to cookies....

class Category_Model extends CI_Model {
    function get_category($parent) {
        $this->db->select('category_id,category,parent');
        $this->db->where('parent',$parent);
        $this->db->from('category');
        $query = $this -> db -> get();
        return $query->result();
    }
}