查阅好多，终于找到解决方法了

$postStr = $GLOBALS ["HTTP_RAW_POST_DATA"];

$postStr = file_get_contents("php://input");//因为很多都设置了register_globals禁止,不能用$GLOBALS["HTTP_RAW_POST_DATA"];

下面附上测试好的代码

  this->_helper->viewRenderer->setNoRender ();
        //get post data, May be due to the different environments
        //$postStr = $GLOBALS ["HTTP_RAW_POST_DATA"];
        $postStr = file_get_contents("php://input");
        
        //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
                    
                    
                    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...";
            }
        
        }
        else
        {
            echo "apibug Input something...";
            exit ();
        }
    }