Redirect new members and existing members that changes their existing level back to the referring page after checkout.

my-pmpro-redirect-to-referring-page-after-checkout.php

<?php
/**
 * This gist is a placeholder for historic purposes.
 * And updated and improved version may be found here:
 * @link https://gist.github.com/ipokkel/32d6ba9425d4550a5861709498e00135
 */


// Original recipe with improved logic to avoid redirecting to PMPro pages:

// -------------------------------------------------------------------------
/**
 * Display the last visited non-PMPro page in the confirmation message, OR
 * redirect the member back to the last visited page.
 *
 * Usage Guide:
 *
 * 1. Set your preference to either display the link or redirect back to
 *    the referrer page by setting the $toggle_display_redirect variable
 *    to true or false.
 * 2. Customize your message for the confirmation page in the $referrer_message
 *    variable.
 *
 * This recipe requires that the Message for Logged-out Users  and
 * Message for Logged-in Non-members setting on the
 * Advanced Settings page (Memberships > Settings > Advanced) has
 * "?redirect_to=!!referrer!!" after the !!levels_page!! placeholder.
 *
 * E.g <a href="!!levels_page_url!!?redirect_to=!!referrer!!">Join Now</a>, or
 * <a href="!!login_url!!?redirect_to=!!referrer!!">Login</a> <a href="!!levels_page_url!!">Join Now</a>
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_display_link_or_redirect_to_last_page_after_checkout() {

	// set to true to display the link in the confirmation message OR set to false to redirect.
	$toggle_display_redirect = true;

	// Bail if PMPro or required custom functions not available.
	if ( ! function_exists( 'pmpro_hasMembershipLevel' ) || ! function_exists( 'my_pmpro_add_previous_page_link_to_confirmation_message' ) || ! function_exists( 'my_pmpro_confirmation_redirect_to_last_post_viewed' ) ) {
		return;
	}

	if ( $toggle_display_redirect ) {
		add_filter( 'pmpro_confirmation_message', 'my_pmpro_add_previous_page_link_to_confirmation_message', 20, 1 );
	} else {
		add_filter( 'pmpro_confirmation_url', 'my_pmpro_confirmation_redirect_to_last_post_viewed', 10, 3 );
	}
}
add_action( 'init', 'my_pmpro_display_link_or_redirect_to_last_page_after_checkout' );

/*
 * Remember referring page
 */
function last_post_viewed_cookie() {

	global $wp;
	$rurl = '';

	if ( isset( $_REQUEST['redirect_to'] ) && ! empty( $_REQUEST['redirect_to'] ) ) {
		$rurl = $_REQUEST['redirect_to'];
	} else {
		$rurl = home_url( add_query_arg( array(), $wp->request ) );
	}
	if ( empty( $rurl ) ) {
		return;
	}

	global $post, $pmpro_pages;

	//don't track pmpro pages
	if ( in_array( strval( url_to_postid( $rurl ) ), $pmpro_pages, true ) ) {
		return;
	}

	setcookie( 'last_post_viewed', $rurl, null, '/' );

}
add_action( 'wp', 'last_post_viewed_cookie' );


/*
 * Add last page link to confirmation page text.
 */
function my_pmpro_add_previous_page_link_to_confirmation_message( $message ) {

	$url = false;

	if ( isset( $_COOKIE['last_post_viewed'] ) && ! empty( $_COOKIE['last_post_viewed'] ) ) {
		$url = $_COOKIE['last_post_viewed'];
	}

	// Add URL to message if it's considered safe.
	if ( wp_http_validate_url( $url ) ) {

		// Customize your message here
		/* translators: %s: URL of last visited page */
		$referrer_message = '<p>' . sprintf( __( 'View your last visited page here: %s.', 'pmpro-customizations' ), '<a href="' . $url . '">' . $url . '</a>' ) . '</p>';

		// Add  to the message
		$message .= $referrer_message;
	}

	return $message;
}

/*
 * Redirect to last page instead of confirmation page after checkout.
 */
function my_pmpro_confirmation_redirect_to_last_post_viewed( $url, $user_id, $level ) {
	if ( isset( $_COOKIE['last_post_viewed'] ) && ! empty( $_COOKIE['last_post_viewed'] ) ) {
		$new_url = $_COOKIE['last_post_viewed'];
	}

	// Change confirmation URL if it's considered safe.
	if ( wp_http_validate_url( $new_url ) ) {
		$url = $new_url;
	}

	return $url;
}

Reviewed/updated gist here: sc0ttkclark/c46cc0f1abdafd07a853787c858badfe

Some users reported the recipe not redirecting after checkout.
Updated working gist here: https://gist.github.com/ipokkel/32d6ba9425d4550a5861709498e00135