/**
  * Prepend a path to relative URIs in CSS files
  *
  * @param string $css
  * @param string $path The path to prepend.
  * @param integer $browserCacheId
  * @param array $browserCacheExtensions
  *
  * @return string
  */
 private static function _prepend($css, $path, $browserCacheId = 0, $browserCacheExtensions = array())
 {
     self::$_prependRelativePath = $path;
     self::$_browserCacheId = $browserCacheId;
     self::$_browserCacheExtensions = $browserCacheExtensions;
     $css = self::_trimUrls($css);
     // append
     $css = preg_replace_callback('/@import\\s+([\'"])(.*?)[\'"]/', array(self::$className, '_processUriCB'), $css);
     $css = preg_replace_callback('/url\\(\\s*([^\\)\\s]+)\\s*\\)/', array(self::$className, '_processUriCB'), $css);
     return $css;
 }