get subcategory by category id opencart

============================1st subcategory=============================
  <?php
            $this->load->model('catalog/category');
           $categories_1 = $this->model_catalog_category->getCategories(59);  
           if ($categories_1) {  

           foreach ($categories_1 as $category_1) {
           $output .= '<li>';
      $unrewritten  = $this->url->link('product/category', 'path=' . $category_1['category_id']);
      $output .= '<a href="'.($unrewritten).'">' . $category_1['name'] . '</a>';
           $output .='</li>';
           }
           }
   
  echo $output;
 ?>


========================2nd and 3rd subcategory============================

<div class="box">
  <div class="box-heading"><?php echo $heading_title; ?></div>
  <div class="box-content">
  <div class="box-category">
    <?php
$this->load->model('catalog/category');
$this->load->model('catalog/product');

$categories_1 = $this->model_catalog_category->getCategories(0);  
if ($categories_1) {$output = '<ul class="glossycategory" id="verticalmenu">';}  

foreach ($categories_1 as $category_1) {
$output .= '<li>';
$unrewritten  = $this->url->link('product/category', 'path=' . $category_1['category_id']);
$output .= '<a href="'.($unrewritten).'">' . $category_1['name'] . '</a>';

$categories_2 = $this->model_catalog_category->getCategories($category_1['category_id']);

if ($categories_2) {$output .= '<ul>';}

foreach ($categories_2 as $category_2) {
$output .= '<li>';
$sub_unrewritten = $this->url->link('product/category', 'path=' . $category_1['category_id'] . '_' . $category_2['category_id']);
$output .= '<a href="'.($sub_unrewritten).'">'. $category_2['name'] . '</a>';

$categories_3 = $this->model_catalog_category->getCategories($category_2['category_id']);

if ($categories_3) {$output .= '<ul>';}

foreach ($categories_3 as $category_3) {
$output .= '<li>';
$third_sub_unrewritten = $this->url->link('product/category', 'path=' . $category_1['category_id'] . '_' . $category_2['category_id'] . '_' . $category_3['category_id']);
$output .= '<a href="'.($third_sub_unrewritten).'">'.$category_3['name'].'</a>';
$output .= '</li>';
}

if ($categories_3) {$output .= '</ul>';}
$output .= '</li>';
}
if ($categories_2) {$output .= '</ul>';}
$output .= '</li>';
}
if ($categories_1) {$output .= '</ul>';}
echo $output;
?>
  </div>
  </div>
</div>