PHP中的多用户电子商店（Codeigniter）

I am creating an e-shop in PHP using Codeigniter framework but I am stacked, how to design the database. My idea is, that this e-shop will have a backend full of functions and settings (creating categories, colours, adding goods, filemenager functions, and many many more). But, every user has his/her own categories, own colours, own goods,... so perhaps nothing is in common between users. Users have unique u_ids. Should I add a user ID to every table that I am using ? I give you an example of what I am intenting to do:

public function add_type_by_e_id($data = array(), $e_id = 0) {
    if (!is_numeric($e_id)) {
        $e_id = 0;
    }
    if (is_array($data) && count($data) > 0) {
        $data['e_id'] = $e_id;
        $this->db->insert($this->types, $data);
        return $this->db->insert_id();
    }
    return false;
}

public function get_all_types_by_e_id($e_id = 0) {
    if (!is_numeric($e_id)) {
        $e_id = 0;
    }
    $this->db->from($this->types);
    $this->db->where('e_id', $e_id);
    $this->db->order_by('type', 'ASC');
    $query = $this->db->get();

    $result = $query->result();
    $query->free_result();

    if (count($result) > 0) {
        return $result;
    }
    return false;
}

public function get_type_by_t_id_e_id($t_id = 0, $e_id = 0) {
    if (!is_numeric($e_id)) {
        $e_id = 0;
    }
    if (is_numeric($t_id) && $t_id > 0) {
        $this->db->from($this->types);
        $this->db->where('t_id', $t_id);
        $this->db->where('e_id', $e_id);
        $query = $this->db->get();

        $result = $query->result();
        $query->free_result();

        if (count($result) > 0) {
            return $result[0];
        }
    }
    return false;
}

(In fact, $e_id is the user identifier.) My question is, whether is this the best and simplest way. The idea for this "$e_id thing" is, that I am afraid, that if no $e_id in this condition will be present, some user (or maybe a bug in the application) can fetch the data of the other user.

Thank you and I am waiting for your questions, because I feel it is a little bit confusing. :)

Peter