//maps any absolute url from this drupal site to nid if applicable.
//can also be used to check whether an absolute path is in the site and points to a node (e.g. node/1)
function _pingback_url_to_nid($url) {
  //first check if the url is really in our site, as well as getting the non-base-url part
  if (preg_match($a = '#^'. preg_quote($GLOBALS['base_url'], '#') .'/(.+)$#', $url, $matches)) {
    //dpm($matches[1]);
    //dpm(drupal_get_normal_path($matches[1]));
    
    if (!variable_get('clean_url', 0)) {
      // Clean URLs not enabled. Strip '?q=' from URL.
      $matches[1] = str_replace('?q=', '', $matches[1]);
    }

    if (
      variable_get('language_count', 1) > 1 &&
      variable_get('language_negotiation', LANGUAGE_NEGOTIATION_NONE) != LANGUAGE_NEGOTIATION_NONE
    ) {
      // Check if we have a language prefix and strip it
      $alias = explode('/', $matches[1]);
      $alias_lang = language_default('language');
      $__langs = language_list();
      foreach (array_keys($__langs) as $lang) {
        if ($lang == $alias[0]) {
          $alias_lang = $lang;
          array_shift($alias);
          break;
        }
      }
      $matches[1] = implode('/', $alias);
      $path = drupal_get_normal_path($matches[1], $alias_lang);
    } else {
      $path = drupal_get_normal_path($matches[1]);
    }
    
    if (preg_match($b = '#^node/([0-9]+)$#', $path, $matches2)) {
      return $matches2[1];
    } //else dpm($b);
  }
  
  //dpm($a);
  return FALSE;
}