Error when redirecting to another page?

Question

PHP

Error when logging in

Warning: Cannot modify header information - headers already sent by (output started at C:\OpenServer\domains\localhost\login.php:2) in C:\OpenServer\domains\localhost\login.php on line 29

As soon as I insert a piece of code for redirection on line 1 or 2 in the editor, there is no error, but somewhere else if I insert it, then an error.
piece of code

header('Location: /');
exit();

Answer 1

If you want to work with headers, then you cannot place bare html with php code inclusions, the log will be clogged with warnings.
When executing a php file at the beginning, it should be silent like a fish, if the logic of work provides for a choice - work first with headers, and then display content, or simply output content (via print, echo, etc.).
That is, headers must fire first, any echo between header() will break the order.