Multi Level Nested Category System in Codeigniter and MySql

Ref : https://www.myprogrammingtutorials.com/multi-level-nested-category-system-codeigniter-mysql.html

CREATE TABLE `categories` (
  `cat_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `cat_name` varchar(50) COLLATE latin1_general_ci NOT NULL,
  `parent_id` int(10) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`cat_id`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci;

Model Function

public function get_categories(){

        $this->db->select('*');
        $this->db->from('categories');
        $this->db->where('parent_id', 0);

        $parent = $this->db->get();

        $categories = $parent->result();
        $i=0;
        foreach($categories as $p_cat){

            $categories[$i]->sub = $this->sub_categories($p_cat->cat_id);
            $i++;
        }
        return $categories;
    }

    public function sub_categories($id){

        $this->db->select('*');
        $this->db->from('categories');
        $this->db->where('parent_id', $id);

        $child = $this->db->get();
        $categories = $child->result();
        $i=0;
        foreach($categories as $p_cat){

            $categories[$i]->sub = $this->sub_categories($p_cat->cat_id);
            $i++;
        }
        return $categories;
    }