设置php中的函数路径

我有gcm在文件夹(dirC)…这里的路径

index.php
dirA/    
dirB/    
dirC/GCM.php    
----/config.php
----/sendMsg.php
dirD/
dirE/dirE1/test.php            //send from here
dirF/

gcm.php有如下代码

class GCM {
    function __construct() {
    }
    /**
     * Sending Push Notification
     */
    public function send_notification($registatoin_ids, $message) {
        // include config
        include_once './config.php';
          //other code here
        echo 'test';
     }
}

和i成功地从sendMsg.php发送消息，这里的代码

include_once './GCM.php';
    $message="hello word";
    $gcm = new GCM();
    $registatoin_ids = array("SEJFOJOEUJFPUWPJR0923740JEU092308UPUPUAUOJLJLJJVPW634");
    $message = array($message);
    $result = $gcm->send_notification($registatoin_ids, $message);
    echo $result;

我的问题是如何设置路径使用include_once从test.php与GCM.php通信?

here test.php code

include_once './GCM.php'; //my proble is here ???????????
$message="hello word";
$gcm = new GCM();
$registatoin_ids = array("SEJFOJOEUJFPUWPJR0923740JEU092308UPUPUAUOJLJLJJVPW634");
$message = array($message);
$result = $gcm->send_notification($registatoin_ids, $message);
echo $result;

thanks in advance.