Automatically add terms to a menu on Wordpress

auto-add-terms-to-menu.php

<?php

/**
 * Display Categories on the Primary menu
 */
add_filter('wp_get_nav_menu_items', function ($items, $menu, $args) {

	$primary_menu_id = 47; // the id of the main menu;
	$shop_menu_item_id = 1248; // the id of the main item that serves as the parent;
	$taxonomy_name = 'product_cat'; // the machine name of the vocavulary you want to use;

	if( $menu->term_id != $primary_menu_id ) {
		return $items;
	}

	// don't add child categories in administration of menus
	if ( is_admin() ) {
		return $items;
	}

	$terms = get_terms([
		'taxonomy' => $taxonomy_name,
		'hide_empty' => false,
	]);

	foreach ( $items as $index => $i ) {

		if ( $shop_menu_item_id !== $i->ID ) {
			continue;
		}

		foreach ( $terms as $index2 => $term ) {

			if ( $term->slug == 'uncategorized' ) {
				continue; // remove the uncategorized term
			}

			if ( $term->parent != 0 ) {
				continue; // remove children as this stage
			}

			$url = get_term_link( $term );

			$e = new \stdClass();

			$e->title = $term->name;
			$e->url = $url;
			$e->menu_order = 100 * ( $term->term_order + 1 ) + $index2;
			$e->post_type = 'nav_menu_item';
			$e->post_status = 'published';
			$e->post_parent = $i->ID;
			$e->menu_item_parent = $i->ID;
			$e->type = 'custom';
			$e->object = 'custom';
			$e->description = '';
			$e->object_id = 0;
			$e->db_id = 0;
			$e->ID = 0;
			$e->classes = array();

			$items[] = $e;

			$term_children = get_term_children( $term->term_id, $taxonomy_name );

			// add child categories

			foreach ( $term_children as $index3 => $child_id ) {

				$child = get_term( $child_id );

				if ( $child->count == 0 ) {
					continue;
				}

				$url = get_term_link( $child );

				$e = new \stdClass();

				$e->title = $child->name;
				$e->url = $url;
				$e->menu_order = 200 * ( $child->term_order + 1 ) + $index3;
				$e->post_type = 'nav_menu_item';
				$e->post_status = 'published';
				$e->post_parent = $term->ID;
				$e->menu_item_parent = $term->ID;
				$e->type = 'custom';
				$e->object = 'custom';
				$e->description = '';
				$e->object_id = 0;
				$e->db_id = 0;
				$e->ID = 0;
				$e->classes = array();

				$items[] = $e;
			}
		}
	}
	return $items;
}, 10, 3);