What is the purpose of the "header()" function?

Question

In my question about the HTTP protocol understood that HTTP requests have a format that is described by the protocol itself.

However, in PHP there is a function that sends data to HTTP which is the function header() and I did not understand very well its purpose, even reading the manual I was left with doubts that can be clarified with a canonical answer.

Doubts

1. What is the purpose of the function header()?
2. The function header() has some relation to the HTTP protocol?
3. When the function header() must be used?

^{To further facilitate my understanding, I would like practical examples if possible.}

Answer 1

What is the purpose of the header function()?

Just mount a header for the HTTP package that will be transmitted in that request. By default it generates a basic with the minimum necessary, if you want to customize can use this function.

The header() function has some relation to the HTTP protocol?

Totally, it just makes sense.

When the header() function should be used?

Whenever you need to do something that is not common. You can send a specific error (HTTP/1.0 404 Not Found), can indicate the type of content that will be sent (Content-Type), expiration (Expires), work with authentication (WWW-Authenticate: Basic realm="realm"), and perhaps one of the most used in practice, redirect to other content (location). But anything that allows the protocol can be used, even header extensions.

Documentation.

Examples:

header('HTTP/1.1 301 Moved Permanently');
header('Location: http://www.example.com');

header('HTTP/1.1 503 Service Temporarily Unavailable');
header('Status: 503 Service Temporarily Unavailable');
header('Retry-After: 60');

header('Content-Type: image/png');

header('Expires: Sat, 26 Jul 1997 05:00:00 GMT');
header('Cache-Control: no-store, no-cache, must-revalidate');
header('Cache-Control: pre-check=0, post-check=0, max-age=0');
header ('Pragma: no-cache');

header('Content-Disposition: attachment; filename=' . urlencode($file));   
header('Content-Type: application/force-download');
header('Content-Type: application/octet-stream');
header('Content-Type: application/download');
header('Content-Description: File Transfer');            
header('Content-Length: ' . filesize($file));
echo file_get_contents($file);

header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="plain_text_file.txt"');

header('WWW-Authenticate: Basic realm="The Realm"');
header('HTTP/1.0 401 Unauthorized');

I put in the Github for future reference.

Answer 2

The Header of an HTTP request/response is a key/value list.

1 - Basically this function allows you to add values, or update values that already exist in the default response header (defined by the server).

2 - Yes, its function is to edit the header of an HTTP response.

3 - When you need to enter/change some response header value. This can be useful for many things, for example: Force redirect; Force download of content; Return a specific status; etc.

http://php.net/manual/en/function.header.php

Answer 3

Just to reinforce what has already been said, the function header mounts the http response header that will be sent to the browser.

An addendum on function header is that it has an optional second parameter, called $replace. If you used as false, the header can be repeated, as in the example below:

header('WWW-Authenticate: Negotiate');
header('WWW-Authenticate: NTLM', false);