Make WordPress relative URLS

path-helper.php

<?php

function make_paths_relative($buffer) {
  $home_url = esc_url(home_url('/'));
  $home_url_relative = wp_make_link_relative($home_url);

  $home_url_escaped = str_replace('/', '\/', $home_url);
  $home_url_escaped_relative = str_replace('/', '\/', $home_url_relative);

  $buffer = str_replace($home_url, $home_url_relative, $buffer);
  $buffer = str_replace($home_url_escaped, $home_url_escaped_relative, $buffer);

  return $buffer;
}

function buffer_start_relative_paths() {
  ob_start('make_paths_relative');
}
function buffer_end_relative_paths() {
  @ob_end_flush();
}

add_action('registered_taxonomy', 'buffer_start_relative_paths');
add_action('shutdown', 'buffer_end_relative_paths');