第一个公众微信demo老是运行不成功

<?phpdefine("TOKEN", "imates");//自己定义的token 就是个通信的私钥$wechatObj = new wechatCallbackapiTest();$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 = "<xml>            <ToUserName><![CDATA[%s]]></ToUserName>            <FromUserName><![CDATA[%s]]></FromUserName>            <CreateTime>%s</CreateTime>            <MsgType><![CDATA[%s]]></MsgType>            <Content><![CDATA[%s]]></Content>            <FuncFlag>0<FuncFlag>            </xml>";            if(!empty( $keyword ))            {                $msgType = "text";                $contentStr = '你好啊，?丝';                $resultStr = sprintf($textTpl, $fromUsername, $toUsername, $time, $msgType, $contentStr);                echo $resultStr;            }else{                echo '咋不说哈呢';            }        }else {            echo '咋不说哈呢';            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;        }    }}?>