代码：

[root@docker-server]# cat index.php

<?php
session_start();
header("by: aaa");

Warning: session_start(): Session cannot be started after headers have already been sent in D:\phpstudy_pro\WWW\pt\t.php on line 3
在调用session_start()前不能有任何输出,包括空格和空行,否则会报上面错。

Warning: Cannot modify header information – headers already sent by (output started at D:\phpstudy_pro\WWW\pt\t.php:1) in D:\phpstudy_pro\WWW\pt\t.php on line 4
在调用 header()前不能有任何输出,包括空格和空行,否则会报上面错。