I found a solution through the answers to the above questions.
function my_menu_parent($theme_location) {
$locations = get_nav_menu_locations();
if ( isset( $locations[ $theme_location ] ) ) {
$menu = wp_get_nav_menu_object( $locations[ $theme_location ] );
$menu_items = wp_get_nav_menu_items($menu->term_id);
_wp_menu_item_classes_by_context( $menu_items );
$breadcrumbs = array();
foreach ( $menu_items as $menu_item ) {
if ($menu_item->current_item_ancestor) {
$breadcrumbs[] = $menu_item->title;
}
}
return $breadcrumbs;
}
}
<?php
$parentitems = my_menu_parent( 'menu-1' );
foreach ( $parentitems as $parentitem ) {
echo $parentitem."<br>";
}