checkbox

Hello,
I use the CodeIgniter 3.0.1 (is too old for upgrade and I have lots of problems after it)

So, this is the edit page of the product, the array is working insert on my db like as: a:1:{i:0;s:2:"1 ";} but if I try to edit again the checkbox is not are selected.

my Controllers:

PHP Code:

public function edit($id) {
        if ($this->input->post('name')) {
            $data['name'] = $this->input->post('name');
            $data['model'] = $this->input->post('model');
            $data['category_id'] = serialize($this->input->post('category_id'));
            $data['brand_id'] = $this->input->post('brand_id');
            $data['price'] = $this->input->post('price');
            $data['tag_line'] = $this->input->post('tag_line');
            $data['features'] = $this->input->post('features');
 
            $this->product->update($data, $id);

            redirect('/admin/products', 'refresh');
        }

        $product = $this->product->get($id);
        $brands = $this->brand->get_all();
        $categories = $this->category->get_all();

        $data['brands'] = $brands;
        $data['categories'] = $categories;
        $data['product'] = $product;
        $data['page'] = $this->config->item('ci_my_admin_template_dir_admin') . "products_edit";
        $this->load->view($this->_container, $data);
    } 


View page:

PHP Code:

<?php
$i = 0;
foreach($categories as $key => $category)
{
?>
  <label>
          <input type="checkbox" name="category_id[]" value="<?= $category['id'] ?> <?= ($product->category_id == $category['id']) ? 'checked="checked"' : ''?>"> <?= $category['name'] ?></input>
           <br />
   </label>
 <?php
$i++;
}
?>

get_all function:

PHP Code:

public function get_all($fields = '', $where = array(), $table = '', $limit = '', $order_by = '', $group_by = '') {
        $data = array();
        if ($fields != '') {
            $this->db->select($fields);
        }

        if (count($where)) {
            $this->db->where($where);
        }

        if ($table != '') {
            $this->table_name = $table;
        }

        if ($limit != '') {
            $this->db->limit($limit);
        }

        if ($order_by != '') {
            $this->db->order_by($order_by);
        }

        if ($group_by != '') {
            $this->db->group_by($group_by);
        }

        $Q = $this->db->get($this->table_name);

        if ($Q->num_rows() > 0) {
            foreach ($Q->result_array() as $row) {
                $data[] = $row;
            }
        }
        $Q->free_result();

        return $data;
    } 


What I can do it?