Converts all links in post content, attachments etc. to root relative. This is specifically designed to work well with RAMP

relative-urls.php

<?php
/**
 * Plugin Name: Relative URLs
 * Description: Converts all links in post content, attachments etc. to root
 *              relative.  This is specifically designed to work well with
 *              <a href="https://crowdfavorite.com/ramp/">RAMP</a>.
 * Version:     1.1
 * Author:      Zachary Scott
 */

namespace zacscott;

/**
 * Relative links plugin driver class.
 *
 * @author Zachary Scott <zscott.dev@gmail.com>
 */
class RelativeLinksPlugin {

	function __construct() {
		add_filter( 'content_save_pre', array( $this, 'make_root_relative' ) );
	}

	// Makes all of the URLs in the given string root relative for the current site
	function make_root_relative( $string ) {
		assert( is_string( $string ) );

		// The site URLs to trim
		$site_urls = array( home_url(), );
		$site_urls = apply_filters( 'relative_urls_list', $site_urls );

		// Trim each of the site URLs in hte given content
		foreach ( $site_urls as $site_url ) {

			// Remove the path from the url
			$url_bits = parse_url( $site_url );
			$url_bits['path'] = '';

			// Make URL relative in string content (for both http & https)
			foreach ( array( 'http', 'https' ) as $scheme ) {

				$url_bits['scheme'] = $scheme;
				$site_url = $this->unparse_url( $url_bits );

				$string = str_replace( $site_url, '', $string );

			}

		}

		return $string;

	}

	// Unparses a URL generated by `parse_url()`
	// based off this - http://php.net/manual/en/function.parse-url.php#106731
	function unparse_url( $parsed_url ) {
		assert( ! empty( $parsed_url ) );

		$scheme   = isset( $parsed_url['scheme'] )   ? $parsed_url['scheme'] . '://' : '';
		$host     = isset( $parsed_url['host'] )     ? $parsed_url['host']           : '';
		$port     = isset( $parsed_url['port'] )     ? ':' . $parsed_url['port']     : '';
		$user     = isset( $parsed_url['user'] )     ? $parsed_url['user']           : '';
		$pass     = isset( $parsed_url['pass'] )     ? ':' . $parsed_url['pass']     : '';
		$pass     = ( $user || $pass )  ? "$pass@" : '';
		$path     = isset( $parsed_url['path'] )     ? $parsed_url['path']           : '';
		$query    = isset( $parsed_url['query'] )    ? '?' . $parsed_url['query']    : '';
		$fragment = isset( $parsed_url['fragment'] ) ? '#' . $parsed_url['fragment'] : '';

		return "$scheme$user$pass$host$port$path$query$fragment";

	}

}

// Boot
new RelativeLinksPlugin();