<?php
add_filter( 'rwmb_meta_boxes', function( $meta_boxes ) {
	$meta_boxes[] = [
		'title' => 'Test Meta Box',
		'fields' => [
			[
				'type' => 'select_advanced',
				'id'   => 's',
				'name' => 'Select a month',
				'options' => [
					'jan' => 'January',
					'feb' => 'February',
				],
			],
			[
				'type' => 'group',
				'id'   => 'g_jan',
				'name' => 'January Options',
				'visible' => ['s', 'jan'],
				'fields' => [
					[
						'id'   => 'l_jan',
						'name' => 'Location',
					],
				],
			],
			[
				'type' => 'group',
				'id'   => 'g_feb',
				'name' => 'February Options',
				'visible' => ['s', 'feb'],
				'fields' => [
					[
						'id'   => 'l_feb',
						'name' => 'Location',
					],
				],
			],
		],
	];
	return $meta_boxes;
} );