作为一个纯理论的例子，我将这样处理这个问题：

$cats = get_categories();
echo \'<ul>\';
foreach($cats as $cat) {
    echo\'<li>\'.$cat->name;
    if($cat->parent != 0) {
       $subcats = get_category(\'child_of=\'.$cat->cat_ID;
       echo \'<ul>\';
       foreach($subcats as $subcat){
        echo \'<li>\'.$subcat->name.\'</li>\';
       }
    }
    echo \'</li>\';
}
echo \'</ul>\';