Headers already sent [ 2.0 版本 ]

控制器当中echo或者var_dump获取到的数据的时候，数据没有全部输出，出现报错如下：

An Error occurred while handling another error:
yii\web\HeadersAlreadySentException: Headers already sent

此问题多处出现，请求大神指导

完整报错如下

An Error occurred while handling another error:
yii\web\HeadersAlreadySentException: Headers already sent in 
D:\phpStudy\WWW\fccms\backend\controllers\TestController.php on line 40.

in D:\phpStudy\WWW\fccms\vendor\yiisoft\yii2\web\Response.php:366

Stack trace:
#0 D:\phpStudy\WWW\fccms\vendor\yiisoft\yii2\web\Response.php(339):
yii\web\Response->sendHeaders()

#1 D:\phpStudy\WWW\fccms\vendor\yiisoft\yii2\web\ErrorHandler.php(135):
yii\web\Response->send()

#2 D:\phpStudy\WWW\fccms\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 
D:\phpStudy\WWW\fccms\backend\controllers\TestController.php on line 40. 
in D:\phpStudy\WWW\fccms\vendor\yiisoft\yii2\web\Response.php:366

Stack trace:
#0 D:\phpStudy\WWW\fccms\vendor\yiisoft\yii2\web\Response.php(339): yii\web\Response->sendHeaders()

#1 D:\phpStudy\WWW\fccms\vendor\yiisoft\yii2\base\Application.php(392): yii\web\Response->send()

#2 D:\phpStudy\WWW\fccms\backend\web\index.php(17): yii\base\Application->run()

#3 {main}