WordPress Plugin: Redirect to the newest or the latest post via a GET parameter

simple-redirections.php

<?php
/**
 * Plugin Name: Simple Redirections
 * Plugin URI:  https://gist.github.com/birgire/b1c8b3b94da4ee2f94ba
 * Author:      Birgir Erlendsson (birgire)
 * Version:     0.0.1
 * Licence:     GPLv2+
 * Description: Redirect to the newest or the latest post with a GET parameter
 */

/** 
 * Usage example:
 *
 *   example.com?redirect=oldest  (redirect to the oldest post)
 *   example.com?redirect=newest  (redirect to the newest post)
 */

add_action( 'template_redirect', function(){
    $redirect = filter_input( INPUT_GET, 'redirect', FILTER_SANITIZE_STRING );
    if( in_array( $redirect, array( 'newest', 'oldest' ) ) )
    {
        $post_ids = get_posts(
            array(
               'posts_per_page' => 1, 
               'orderby'        => 'date',
               'order'          => ( 'latest' === $redirect ) ? 'DESC' : 'ASC',
               'fields'        => 'ids',
            )   
        );
        if( isset( $post_ids[0] ) )
            wp_safe_redirect( get_permalink( $post_ids[0] ) ); exit();
             	
    }
});