Parent class for WP Menu item

wpnav-menu.php

<?php

/* Parent class for menu item */
function rtp_add_menu_parent_class( $items ) {

	$parents = array();
	foreach ( $items as $item ) {
		if ( $item->menu_item_parent && $item->menu_item_parent > 0 ) {
			$parents[] = $item->menu_item_parent;
		}
	}

	foreach ( $items as $item ) {
		if ( in_array( $item->ID, $parents ) ) {
			$item->classes[] = 'menu-parent-item rtp-parent-menu';
		}
	}

	return $items;
}
add_filter( 'wp_nav_menu_objects', 'rtp_add_menu_parent_class' );