private static function _checkClientCache()
 {
     require_once 'HTTP/ConditionalGet.php';
     $cgOptions = array('lastModifiedTime' => 0, 'isPublic' => true, 'encoding' => '', 'maxAge' => 1800);
     $cg = new HTTP_ConditionalGet($cgOptions);
     if (MINCO_CLIENT_CACHE && $cg->cacheIsValid) {
         // client's cache is valid
         $cg->sendHeaders();
         return true;
     } else {
         // client will need output
         self::$_headers = $cg->getHeaders();
         unset($cg);
     }
     return false;
 }