Question

I know in PHP, it sends the X-Powered-By header to have the PHP version.

I also know by appending some checksums, you can get access to PHP’s credits, and some random images (more info here).

I also know in php.ini you can turn expose_php = off.

But here is something I have done on a few sites, and that is use

header('X-Powered-By: Alex');

When I view the headers, I can see that it is now ‘Alex’ instead of the PHP version. My question is, will this send the previous PHP header first (before it reaches my header(), and is it detectable by any sniffer program? Or are headers ‘collected’ by PHP, before being sent back to the browser?

By the way, this is not for security by obscurity, just curious how headers work in PHP.

Answer 1

In PHP, headers aren’t sent until PHP encounters its first output statement.

This includes anything before the first <?php.

This is also why setcookie sends throws a warning if you try to use it after something has been output:

Warning: Cannot modify header
information – headers already sent by
(output started at
/path/to/php/file.php:100) in
/path/to/php/file.php on line 150

Note that none of this applies if output buffering is in use, as the output will not be sent until the appropriate output buffering command is run.