How do I fix "headers already sent" in PHP?

The error tells you exactly where the output started:

Warning: Cannot modify header information - headers already sent by
(output started at /var/www/config.php:42)

Go to that file and line.

#The usual causes

Whitespace after the closing tag. A newline after ?> at the end of an included file is output.

A BOM at the start of the file. Some editors write a UTF-8 byte order mark, which is three invisible bytes of output. Save as "UTF-8 without BOM".

Debug output you forgot. A stray var_dump or echo earlier in the request.

An error message. With display_errors on, a warning printed earlier counts as output.

#The fix that prevents all of them

Omit the closing ?> in any file that is pure PHP:

<?php
declare(strict_types=1);

function helper(): string {
    return 'ok';
}
// no closing tag — nothing can follow it

This is standard practice in every serious PHP codebase, and PSR-12 requires it.

#And always exit after a redirect

header('Location: /thanks.php');
exit;

header() only queues the redirect. Without exit, the rest of the script still runs — and can still write to your database a second time.