如何在opencart admin / catalog / product_form.tpl上获取所有类别？

I'm using opencart 1.5.6 and what I'm trying to do is add a simple select with all the categories and children categories as option on product_form.tpl.

I tried add to the follow code at admin/controler/catalog/product.php:

    $this->load->model('catalog/category');

    $results = $this->model_catalog_category->getCategories(0);

    foreach ($results as $result) {

        $this->data['categories'][] = array(
            'category_id' => $result['category_id'],
            'name'        => $result['name']
        );
    }

and for the view admin/view/catalog/product_form.tpl:

<tr>
              <td>Categories</td>
              <td><select>
                      <option value="">All Categories</option>
                      <?php foreach ($categories as $category) { ?>
                      <option value="<?php echo $category['name']; ?>"><?php echo $category['name']; ?></option>
                      <?php } ?>
                  </select>
              </td>
          </tr>

but it's not working. It returns nothing.

I wish someone could help me and tell me what I'm missing.

I think Your approach is good but one small mistake - the method getCategories() in administration part does not receive the $parent_id paramaters, but array $data parameter (for sorting and filtering possibilities). So if You want to load the categories in Your product controller, You can proceed like this (I am reusing Your code):

$this->load->model('catalog/category');

$this->data['categories'] = array();

foreach ($this->model_catalog_category->getCategories(array()) as $category) {
    $this->data['categories'][] = array(
        'category_id' => $category['category_id'],
        'name'        => $category['name']
    );
}

And in Your template:

      <?php if($categories) { ?>
      <tr>
          <td>Categories</td>
          <td><select>
                  <option value="">All Categories</option>
                  <?php foreach ($categories as $category) { ?>
                  <option value="<?php echo $category['category_id']; ?>"><?php echo $category['name']; ?></option>
                  <?php } ?>
              </select>
          </td>
      </tr>
      <?php } ?>

I think this is almost the same as what You had, do not understand why it didn't work for You...

The simplest way to get a complete list of categories is to call the database directly, such as

$result = $this->db->query("
    SELECT
        `category_id`,
        `name`
    FROM
        `" . DB_PREFIX. "category_description`
    WHERE
        `language_id` = '" . (int) $this->config->get('config_language_id')  . "'
");

However these will not show the parent categories, or be in any particular order. To do that you would need to run a recursive lookup of $this->model_catalog_category->getCategories(0); changing the 0 to the looped category_id to find it's children, and their children