Replace the content of the Booking Page when a form is successfully submitted.

replace-booking-page-for-rtb.php

<?php
/**
 * Plugin Name: Replace Booking Page Content for Restaurant Reservations
 * Plugin URI: http://themeofthecrop.com
 * Description: Replace the content of the Booking Page when a form is successfully submitted.
 * Version: 0.0.1
 * Author: Theme of the Crop
 * Author URI: http://themeofthecrop.com
 * License:     GNU General Public License v2.0 or later
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU
 * General Public License as published by the Free Software Foundation; either version 2 of the License,
 * or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * You should have received a copy of the GNU General Public License along with this program; if not, write
 * to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
 */
if ( ! defined( 'ABSPATH' ) )
	exit;

if ( !function_exists( 'rtb_replace_booking_page_content' ) ) {
add_filter( 'the_content', 'rtb_replace_booking_page_content', 11 );
function rtb_replace_booking_page_content( $content ) {

	if ( !is_main_query() || !in_the_loop() ) {
		return $content;
	}

	global $rtb_controller;

	$booking_page = $rtb_controller->settings->get_setting( 'booking-page' );
	if ( empty( $booking_page ) ) {
		return $content;
	}

	global $post;
	if ( $post->ID != $booking_page ) {
		return $content;
	}

	if ( $rtb_controller->request->request_inserted !== true ) {
		return $content;
	}

	ob_start();
	?>

	<div class="rtb-message">
		<p><?php echo $rtb_controller->settings->get_setting( 'success-message' ); ?></p>
	</div>

	<?php
	return ob_get_clean();
}
} // endif;