relative-links-zero.php

<?php
/**
 * Plugin Name: Relative Links Zero
 * Description: Makes links relative using WordPress' `wp_make_link_relative()`
 * Version: 1.0.0
 * Author: Neil Sweeney
 * Website: https://wolfiezero.com/
 * License: GPLv2 or later
 * See: https://deluxeblogtips.com/relative-urls/
 */

add_action('template_redirect', 'rl0_relative_urls');

function rl0_relative_urls() {
    // Don't do anything if:
    // - In feed
    // - In sitemap by WordPress SEO plugin
    if (is_feed() || get_query_var( 'sitemap' ))
        return;

    $filters = [
        'post_link',
        'post_type_link',
        'page_link',
        'attachment_link',
        'get_shortlink',
        'post_type_archive_link',
        'get_pagenum_link',
        'get_comments_pagenum_link',
        'term_link',
        'search_link',
        'day_link',
        'month_link',
        'year_link',
        'get_stylesheet_uri',
        'script_loader_src',
        'style_loader_src',
    ];

    foreach ($filters as $filter) {
        add_filter($filter, 'rl0_make_relative');
    }
}

function rl0_make_relative($url) {
    $length = strlen(WP_HOME);

    if (substr($url, 0, $length) === WP_HOME) {
        return str_replace(WP_HOME, '', $url);
    }

    return $url;
}