Root relative asset URLs for WP

root-relative-urls-wp.php

function use_root_relative_url($input) {
  $output = preg_replace_callback(
    '!(https?://[^/|"]+)([^"]+)?!',
    create_function(
      '$matches',
      // if full URL is site_url, return a slash for relative root
      'if (isset($matches[0]) && $matches[0] === site_url()) { return "/";' .
      // if domain is equal to site_url, then make URL relative
      '} elseif (isset($matches[0]) && strpos($matches[0], site_url()) !== false) { return $matches[2];' .
      // if domain is not equal to site_url, do not make external link relative
      '} else { return $matches[0]; };'
    ),
    $input
  );

  /**
   * Fixes an issue when the following is the case:
   * site_url() = http://yoursite.com/inc
   * home_url() = http://yoursite.com
   * WP_CONTENT_DIR = http://yoursite.com/content
   * http://codex.wordpress.org/Editing_wp-config.php#Moving_wp-content
   */
  $str = "/" . end(explode("/", content_url()));
  if (strpos($output, $str) !== false) {
    $arrResults = explode( $str, $output );
    $output = $str . $arrResults[1];
  }

  return $output;
}
if (!is_admin()) {
  add_filter('script_loader_src', 'use_root_relative_url');
  add_filter('style_loader_src', 'use_root_relative_url');
}