Fix WordPress asset URLs when a non-absolute base URL is given

functions.php

<?php
/**
 * Fixes URLs to asset files.
 *
 * If SITE_URL is left blank then WordPress will assume that the SITE_URL is
 * the current URL which causes problems when the current URL is not the root
 * page. So, instead of linking to
 * /wp-content/themes/themename/js/main.js it would link to
 * /some-page-name/a-child-page/wp-content/themes/themename/js/main.js which
 * breaks things. This attemps to fix this.
 *
 * This fix assumes that WordPress is installed in the root web directory for
 * this domain.
 *
 * From: https://gist.github.com/1381015
 *
 * @author Donovan Mueller
 * @param  string $src
 * @return string
 */
function themename_make_script_src_absolute($src)
{   
   // Remove the base_url if present
   $base_url = wp_guess_url();
   $pos = strpos($src, $base_url);
   if ($pos === 0) {
      $src = substr($src, strlen($base_url));
      // If the base URL ended in a GET query string the desired asset SRC will
      // be URL encoded. It must be decoded.
      if (strpos($base_url, '?') !== false) {
         $src = rawurldecode($src);
         // Because this URL was appended to a GET query string if there was a
         // GET query string attached to the asset URL it will start with &
         // instead of ? which causes problems.
         $src = preg_replace('`&`', '?', $src, 1);
      }
   }
   
   // Add a leading slash if not present but should be.
   if (preg_match('~^(?:/|https?://)~i', $src) === 0) {
      $src = "/$src";
   }
   
   return $src;
} // themename_make_script_src_absolute()

add_filter('script_loader_src', 'themename_make_script_src_absolute');