global $cat;
$c = get_category($cat); // 获取当前分类

while ($c->category_parent) { 
    // 如果当前分类存在上级分类，则进入循环
    $c = get_category($c->category_parent); // 获取上级分类
}

return $c->term_id; // 返回父级分类或最顶级分类的ID
    

// 获取父分类ID
global $cat;
$current_cat = get_category($cat);
while ($current_cat->category_parent) {
    $current_cat = get_category($current_cat->category_parent);
}
$parent_cat_id = $current_cat->term_id;

// 获取父分类的所有子分类
$args = array(
    'child_of' => $parent_cat_id,
    'hide_empty' => 0
);
$sub_categories = get_categories($args);

// 循环输出每个子分类的文章
foreach ($sub_categories as $sub_cat) {
    echo '
' . $sub_cat->name . '
';
    $args = array(
        'category__in' => array($sub_cat->term_id),
        'posts_per_page' => -1
    );
    $sub_cat_posts = new WP_Query($args);
    if ($sub_cat_posts->have_posts()) {
        echo '
';
        while ($sub_cat_posts->have_posts()) {
            $sub_cat_posts->the_post();
            echo '
' . get_the_title() . '
';
        }
        echo '
';
    }
    wp_reset_postdata();
}