text / plain内容类型在codeigniter中返回一些<br>或换行符输出

I have the following code in a codeigniter controller:

 $this->output->set_header('Content-type: text/plain; charset=utf-8');

along with

header('Content-type: text/plain; charset=utf-8');

How come I see line breaks in the output?

1-
2-
3-
4- my output