更新1:@birgire 建议更好的方法：

wp_list_categories( [ \'include\' => wp_list_pluck( get_the_category(), \'term_id\' ) ] );

在你的single.php 模板：

$current_cats = get_the_category();
$current_cats_ids = [];

foreach ($current_cats as $cat) {
    $current_cats_ids[] = $cat->term_id;
}

wp_list_categories([
    \'include\' => $current_cats_ids,
]);