Horribly inefficient function to create a menu with all pages as menu items.

menu-item-inserter.php

<?php
/**
 * Insert menu items for all pages.
 *
 * @package BJ\Menu
 * @author bjornjohansen
 * @version 0.1.0
 * @license https://www.gnu.org/licenses/old-licenses/gpl-2.0.html  GNU General Public License version 2 (GPLv2)
 */

/**
 * Insert the menu items for the current page and all subpages.
 *
 * @param  int $menu_id             The menu we’re inserting the menu items into.
 * @param  int $parent_page_id      The current parent page ID.
 * @param  int $parent_menu_item_id The current parent menu item ID.
 */
function bj_insert_menuitems( $menu_id, $parent_page_id, $parent_menu_item_id ) {

	$page_ids = get_posts( [
		'posts_per_page' => 99,
		'post_type' => 'page',
		'post_parent' => $parent_page_id,
		'fields' => 'ids',
	] );

	foreach ( $page_ids as $page_id ) {

		$created_menu_item_id = wp_update_nav_menu_item( $menu_id, 0, [
			'menu-item-title' => get_the_title( $page_id ),
			'menu-item-object-id' => $page_id,
			'menu-item-type' => 'post_type',
			'menu-item-object' => 'page',
			'menu-item-parent-id' => $parent_menu_item_id,
			'menu-item-status' => 'publish',
		] );

		bj_insert_menuitems( $menu_id, $page_id, $created_menu_item_id );

	}

}


/**
 * Run the menu item generation.
 */
add_action( 'shutdown', function() {

	if ( ! isset( $_GET['generate-menu'] ) ) {
		return;
	}

	// Get the menu if it exists.
	$menu_name = 'Main Menu';
	$menu_exists = wp_get_nav_menu_object( $menu_name );

	// If it doesn't exist, let's create it.
	if ( ! $menu_exists ) {
		$menu_id = wp_create_nav_menu( $menu_name );
		$menu_exists = wp_get_nav_menu_object( $menu_name );
	}

	// Create the menu items.
	bj_insert_menuitems( $menu_exists->term_id, 0, 0 );

} );