帮我分析一下这个Php代码，谢谢

/*
    方倍工作室 http://www.cnblogs.com/txw1958/
    CopyRight 2013 www.doucube.com  All Rights Reserved
    这是一个很简单的微信开发程序
*/
//定义TOKEN常量
define("TOKEN", "weixin");
//实例化wechatCallbackapiTest这个类
$wechatObj = new wechatCallbackapiTest();
/**如果有get传参echostr，则调用valid方法，否则调用responseMsg方法。responseMsg方法就是微信自动回复消息
**/
if (isset($_GET['echostr'])) {
    $wechatObj->valid();
}else{
    $wechatObj->responseMsg();
}

//wechatCallbackapiTest类的定义
class wechatCallbackapiTest
{
    public function valid()
    {
        $echoStr = $_GET["echostr"];
        if($this->checkSignature()){
            echo $echoStr;
            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;
        }
    }
    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($keyword == "?" || $keyword == "")
            {
                $msgType = "text";
                $contentStr = date("Y-m-d H:i:s",time());
                $resultStr = sprintf($textTpl, $fromUsername, $toUsername, $time, $msgType, $contentStr);
                echo $resultStr;
            }
        }else{
            echo "";
            exit;
        }
    }
}
?>