Use relative URL for WordPress

wp_relative_url.php

function callback_relative_url($buffer) {
  // Replace normal URLs
  $home_url          = esc_url(home_url('/'));
  $home_url_relative = wp_make_link_relative($home_url);
  
  // Replace URLs in inline scripts
  $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_url() { ob_start('callback_relative_url'); }
function buffer_end_relative_url() { @ob_end_flush(); }

// http://codex.wordpress.org/Plugin_API/Action_Reference
add_action('registered_taxonomy', 'buffer_start_relative_url');
add_action('shutdown', 'buffer_end_relative_url');