1.调用天气api接口
可以在百度apistore找到免费的api接口

//天气api
public function getWeather($city){
    $ch = curl_init();
    $url = 'http://apis.baidu.com/apistore/weatherservice/weather?citypinyin='.$city;
    $header = array(
        'apikey:你自己的apikey'
    );
    // 添加apikey到header
    curl_setopt($ch, CURLOPT_HTTPHEADER  , $header);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    // 执行HTTP请求
    curl_setopt($ch , CURLOPT_URL , $url);
    $res = curl_exec($ch);
    //将json转化为php变量
    $res=json_decode($res);
    return $res;
}

2.获取用户发送过来的数据，并给出回复

//数据载入对象中
//对象名为SimpleXMLElement
$postObj = simplexml_load_string($postStr, 'SimpleXMLElement', LIBXML_NOCDATA);
$fromUsername = $postObj->FromUserName;
$toUsername = $postObj->ToUserName;
$keyword = trim($postObj->Content);
$time = time();
//获取天气的json
$result=(array)self::getWeather("$keyword");
$resStr=(array)$result["retData"];
$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>";
$msgType = "text";
 //将获取的数组连接成自符串
$contentStr ="城市：".$resStr["city"]."\n"."日期：".$resStr["date"]."\n"."时间：".$resStr["time"]."\n"."天气".$resStr["weather"]."\n"."气温：".$resStr["temp"]."\n"."最低温度：".$resStr["l_tmp"]."\n"."最高温度：".$resStr["h_tmp"]."\n"."风向：".$resStr["WD"].$resStr["SD"];
 //封装xml并返回给用户
$resultStr = sprintf($textTpl, $fromUsername, $toUsername, $time, $msgType,$contentStr);
echo $resultStr;