新创建的项目，在controller里面，echo数据报错，是什么问题 [ 新手入门 ]

代码

<?php

namespace frontend\controllers;

class PostController extends BaseController
{

    public function actionIndex()
    {
        echo 123;
    }
}

输出

123

An Error occurred while handling another error:
yii\web\HeadersAlreadySentException: Headers already sent in /Users/gengshaojing/www/basic/frontend/controllers/PostController.php on line 10. in /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/web/Response.php:366
Stack trace:
#0 /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/web/Response.php(339): yii\web\Response->sendHeaders()
#1 /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/web/ErrorHandler.php(135): yii\web\Response->send()
#2 /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/base/ErrorHandler.php(111): yii\web\ErrorHandler->renderException(Object(yii\web\HeadersAlreadySentException))
#3 [internal function]: yii\base\ErrorHandler->handleException(Object(yii\web\HeadersAlreadySentException))
#4 {main}
Previous exception:
yii\web\HeadersAlreadySentException: Headers already sent in /Users/gengshaojing/www/basic/frontend/controllers/PostController.php on line 10. in /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/web/Response.php:366
Stack trace:
#0 /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/web/Response.php(339): yii\web\Response->sendHeaders()
#1 /Users/gengshaojing/www/basic/vendor/yiisoft/yii2/base/Application.php(392): yii\web\Response->send()
#2 /Users/gengshaojing/www/basic/frontend/web/index.php(17): yii\base\Application->run()
#3 {main}