我试图显示一个类别下拉列表（选择），其中父类别和子类别标记在子类别下。示例：

parent A
parent B
parent C
- child of C
- child of C
- child of C
parent D
parent E
- child of E
- child of E
parent F

相反，我得到的是：

parent A
parent B
parent C
parent D
parent E
parent F
- child of C
- child of C
- child of C
- child of E
- child of E

以下是我的短代码：

function categories_dropdown() { 
  $args = array(
  \'taxonomy\'          => \'my_custom_taxonomy\',
  \'hide_empty\'        => false,
  \'echo\'              => true,
  \'hierarchical\'  => true,
  \'show_count\'        => false,
  \'orderby\'           => \'parent\', 
);

$categories = get_categories($args);

$select = "<select name=\'cat\' id=\'categories_dropdown\' class=\'postform\'>n";
$select.= "<option value=\'-1\'>Select category</option>n";

foreach($categories as $category){
  if($category->count > 0){
    if($category->parent!=0) {
      $select.= "<option value=\'".$category->slug."\'> - ".$category->name." (". $category->count .")</option>";
    } else {
      $select.= "<option value=\'".$category->slug."\'>".$category->name." (". $category->count .")</option>";
    }
    
  }
}

$select.= "</select>";

return $select;

}
add_shortcode(\'categories-dropdown\', \'categories_dropdown\');

我这里的问题是，哪些参数负责父分类法类别下的子级的正确排序？这是用参数完成的还是我必须对选项下拉列表的显示进行编码的方式？我真的很感谢你抽出时间。

$args = array(
    \'taxonomy\'     => \'my_custom_taxonomy\',
    \'hide_empty\'   => false,
    \'echo\'         => false,
    \'hierarchical\' => true,
    \'show_count\'   => true,
    \'orderby\'      => \'parent\',
    \'id\'           => \'categories_dropdown\',
    \'value_field\'  => \'slug\',
);

$select = wp_dropdown_categories( $args );