[Wordpress] Automatically set MENU ORDER on create new PAGE. Sources: http://wordpress.stackexchange.com/a/155979/1044, http://wordpress.stackexchange.com/a/155974/1044

wp-admin-newpage-set-menuorder-1.php

<?php
/**
 * Automatically set Menu Order on create new page
 *
 * Source:
 * http://wordpress.stackexchange.com/a/155979/1044
 *
 */
add_action( 'admin_footer-post-new.php', 'my_admin_footer_post_new_php' );

function my_set_menu_order() {
	$ret = array();

	if ( check_ajax_referer('my_set_menu_order_post', 'nonce', false /*die*/) ) {
		global $wpdb;

		// Get the last page order available
		$sql = $wpdb->prepare('SELECT menu_order FROM ' . $wpdb->posts . ' WHERE post_type = %s AND post_status = %s ORDER BY menu_order DESC LIMIT 1', 'page', 'publish' );

		if ( ( $result = $wpdb->get_var( $sql ) ) !== false ) {
			// Set as the new last page order
			$ret['menu_order'] = ++$result;
		}
	}

	header('Content-type: application/json');
	die( json_encode( $ret ) );
}
add_action( 'wp_ajax_my_set_menu_order', 'my_set_menu_order' );

function my_admin_footer_post_new_php() {
?>
<script type="text/javascript">
	jQuery(document).ready(function() {
		(function ($) {
			$.post( ajaxurl, {
				action: 'my_set_menu_order',
				nonce: <?php echo json_encode( wp_create_nonce( 'my_set_menu_order_post' ) ); ?>
			}, function(response) {
				if (response && response.menu_order) {
				$('#pageparentdiv input[name="menu_order"]').val(response.menu_order);
			}
						 }, 'json'
						);
		})(jQuery);
	});
</script>
<?php
}

wp-admin-newpage-set-menuorder-2.php

<?php
/**
 * Automatically set Menu Order on create new page
 * (Alternative method)
 *
 * Source:
 * http://wordpress.stackexchange.com/a/155974/1044
 *
 */
add_action( 'publish_page', 'my_set_to_last_page', 10, 2 );
function my_set_to_last_page( $ID, $post ) {

	if ( ! wp_is_post_revision( $ID ) ){
		if ( $post->menu_order === 0 ) {
			global $wpdb;

			$query = "SELECT menu_order FROM $wpdb->posts WHERE post_type = 'page' AND post_status = 'publish' ORDER BY menu_order DESC LIMIT 1";

			$max_menu_order = $wpdb->get_var( $query );

			$post->menu_order = ++$max_menu_order;

			// unhook this function so it doesn't loop infinitely
			remove_action('publish_page', 'my_set_to_last_page');

			// update the post, which calls save_post again
			wp_update_post( $post );

			// re-hook this function
			add_action('publish_page', 'my_set_to_last_page');
		}
	}
}