Be careful when using WeChat's public account

If you have used a public account, be careful when subscribing to a new user. the previous "Hello2BizUser" text will be pushed and changed to a "subscribe" event. How can I write this in php? I now have a public account, but after someone pays attention to me, I cannot send a message immediately. & Lt ;? Php/** & nbsp; * & nbsp; be careful when wec has used a public account.
When a new user subscribes, the previous "Hello2BizUser" text will be pushed and changed to a "subscribe" event.
How can I write this in php? I now have a public account, but after someone pays attention to me, I cannot send a message immediately.

 /**
  * wechat php test
  */

//define your token
define("TOKEN", "weixin");
$wechatObj = new wechatCallbackapiTest();
$wechatObj->valid();

class wechatCallbackapiTest
{
public function valid()
    {
        $echoStr = $_GET["echostr"];

        //valid signature , option
        if($this->checkSignature()){
        echo $echoStr;
        exit;
        }
    }

    public function responseMsg()
    {
//get post data, May be due to the different environments
$postStr = $GLOBALS["HTTP_RAW_POST_DATA"];

      //extract 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
  
  

  %s
  
  

  
   %s
  
  

  %s
  
  

  %s
  
  

  
   0
  
  

 ";             
if(!empty( $keyword ))
                {
              $msgType = "text";
                $contentStr = "Welcome to wechat world!";
                $resultStr = sprintf($textTpl, $fromUsername, $toUsername, $time, $msgType, $contentStr);
                echo $resultStr;
                }else{
                echo "Input something...";
                }