Submenu disappears when clicked on submenu in wordpress

Here are my costumed functions I wrote for menus and submenus in my wordpress theme, but after I tested it, the submenu disappeared when I clicked on the submenu, because Wordpress does not separate categories from subcategories, so the parameter for this is "cat "which means when I click on the submenu then the function that creates the submenu checks if the cat = id has child categories in the url, but this is not because it is a child category, I am new to wordpress and I don’t know how to deal with it:

function costume_menu() {
$categories =  get_categories('hide_empty=0&style=none&parent=0'); 
  foreach ($categories as $category) {
    (is_category($category->term_id)) ? $active = 'class="active_menu"' : $active = '';
    $nav = '<li>';
    $nav .= '<a '.$active.'href="'.get_category_link($category->term_id).'">'.strtoupper($category->cat_name).'</a>';
    $nav .= '</li>';

    echo $nav;
  }

}

function costume_submenu($cat) {

$categories =  get_categories("child_of=$cat&hide_empty=0"); 
  foreach ($categories as $category) {
    (is_category($category->term_id)) ? $active = 'class="active_menu"' : $active = '';
    $nav = '<li>';
    $nav .= '<a '.$active.'href="'.get_category_link($category->term_id).'">'.strtoupper($category->cat_name).'</a>';
    $nav .= '</li>';

    echo $nav;
  }
}