Upload de várias imagens com Codeigniter salvando apenas um caminho de arquivo no banco de dados MySQL

public function post(){        
    if($this->_validation()===FALSE){ 
        $this->session->set_flashdata('error', 'Ooops, there was an error');
        redirect(base_url("admin/product"));
    }else{
        $files = $_FILES;
        $images = array();
        $cpt = count($_FILES['userfile']['name']);
            for($i=0; $i<$cpt; $i++){
            $_FILES['userfile']['name']= $files['userfile']['name'][$i];
            $_FILES['userfile']['type']= $files['userfile']['type'][$i];
            $_FILES['userfile']['tmp_name']= $files['userfile']['tmp_name'][$i];
            $_FILES['userfile']['error']= $files['userfile']['error'][$i];
            $_FILES['userfile']['size']= $files['userfile']['size'][$i];
            $this->upload->initialize($this->set_upload_options());
            $this->upload->do_upload();
            $images[] = $_FILES['userfile']['name'];
        }
        $fileName = implode(',',$images);

        $data = array(  'kodeProduk'                => $this->input->post('kodeproduk'),
                        'ket'                       => $this->input->post('ket'),

                        'GambarBesar'               => $fileName
        );

        unset($data['submit']);                             
        $this->table->add_record($data);
        $this->session->set_flashdata('success', 'Product has been saved.');
        redirect(base_url("admin/product"));
    }   
}