微信公众平台消息接口(PHP)-官方的Demo有问题

? define("TOKEN", "weixin");
define("MESS","输入点啥吧");
$wechatObj = new wechatCallbackapiTest();
$wechatObj->valid();
//$wechatObj->responseMsg(); 
class wechatCallbackapiTest
{
    public function valid()
    {
        $echoStr = $_GET["echostr"]; 
        if($this->checkSignature()){
            echo $echoStr;
            exit;
        }
    } 
    public function responseMsg()
    {
        $postStr = $GLOBALS["HTTP_RAW_POST_DATA"];
        if (!empty($postStr)){
            $postObj = simplexml_load_string($postStr, 'SimpleXMLElement', LIBXML_NOCDATA);
            $fromUsername = $postObj->FromUserName;
            $toUsername = $postObj->ToUserName;
            $keyword = trim($postObj->Content);
            $time = time();
            $textTpl = "
           
           
            %s
           
           
            0
            ";
            if(!empty( $keyword ))
            {
                $msgType = "text";
                $contentStr = MESS;
                $resultStr = sprintf($textTpl, $fromUsername, $toUsername, $time, $msgType, $contentStr);
                echo $resultStr;
            }else{
                echo MESS;
            } 
        }else {
            echo MESS;
            exit;
        }
    }
 
    private function checkSignature()
    {
        $signature = $_GET["signature"];
        $timestamp = $_GET["timestamp"];
        $nonce = $_GET["nonce"];
        $token =TOKEN;
        $tmpArr = array($token, $timestamp, $nonce);
        sort($tmpArr);
        $tmpStr = implode( $tmpArr );
        $tmpStr = sha1( $tmpStr );
 
        if( $tmpStr == $signature ){
            return true;
        }else{
            return false;
        }
    }
}
    ?>