On WordPress login, redirect to the last post you were viewing.

my_redirect_to_last_post_id.php

/**
 * On login, redirect to the last post viewed.
 * Add this code into a custom plugin.
 * Note that many themes and plugins hook into login_redirect.
 * Whichever hook has the highest priority will be the last call
 * on where to redirect. The priority below is set to 20.
 * Note also that some URLs/pages may note be "posts"
 * and may not have an ID we can track.
 * Note also that some pages are excluded. You may want to exclude other pages.
 * Note also that this may override any specific redirect_to passed in
 * by URL parameter.
 */

/**
 * Remember last post ID
 */
function my_remember_last_post_id() {
	global $pmpro_pages;
	
	$queried_object = get_queried_object();
	
	// don't need to track if already logged in
	if ( is_user_logged_in() ) {		
		return;
	}
	
	// no post?
	if ( empty( $queried_object ) || empty( $queried_object->ID ) ) {
		return;
	}	
	
	// don't track the login page
	if ( function_exists( 'pmpro_is_login_page' ) && pmpro_is_login_page() ) {
		return;
	}
	
	// don't track pmpro pages
	if( is_array( $pmpro_pages ) && in_array($queried_object->ID, $pmpro_pages) ) {
		return;
	}
		
	// Okay set it
	setcookie('last_post_viewed', $queried_object->ID, null, '/');
}
add_action('wp', 'my_remember_last_post_id');

/**
 * On login, redirect to the last post viewed.
 */
function my_redirect_to_last_post_id( $redirect_to, $request, $user ) {	
	// If no redirect already, use the last page viewed
	if ( ! empty( $_COOKIE['last_post_viewed'] ) ) {
		$redirect_to = get_permalink( intval( $_COOKIE['last_post_viewed'] ) );
	}	
	
	return $redirect_to;
}
add_filter('login_redirect', 'my_redirect_to_last_post_id', 20, 3);