为什么CURLOPT_COOKIEJAR以某种方式保存不同的Cookie值而不是Set-Cookie标头？

TL;DR:

I have some very simple PHP code utilizing cURL that makes single HTTP requests (in practice, to a Diaspora* pod, though that shouldn't be relevant to the question). The code takes note of any cookies returned by the web server and then manually sets those values to libcurl's CURLOPT_COOKIE. However, in trying to hunt down a bug, I'm finding that when I use CURLOPT_COOKIEFILE and CURLOPT_COOKIEJAR, the values of the cookies in the cookie file are different than when I use CURLOPT_COOKIE. Why is this the case? (See code below.)

PRIOR RESEARCH

I have already looked other questions such as this one that suggest various ways of manipulating libcurl's options to keep the same resource handle around and the cookies in memory, but this is not suitable to my application. I need to access the cookie values directly and notably not on a filesystem (to save them into a database, but again, this should not matter with regards to the question).

CODE

For completeness, here is a test case for code I am using:

<?php
// This function simply extracts the cookie set by a webserver by looking at the full HTTP source traffic.
function readCookie ($str) {
    $m = array();
    preg_match('/Set-Cookie: (.*?);/', $str, $m);
    return (!empty($m[1])) ? $m[1] : false;
}

// This function does the same for the CSRF token required for login.
function parseAuthenticityToken ($str) {
    $m = array();
    preg_match('/content="(.*?)" name="csrf-token"/', $str, $m);
    return (!empty($m[1])) ? $m[1] : false;
}

// Get first page, to find the CSRF token.
$ch = curl_init('https://diasp.org/');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$resp = curl_exec($ch);
curl_close($ch);

$csrf_token = parseAuthenticityToken($resp);

$params = array(
    'user[username]' => 'my_username',
    'user[password]' => 'my_password',
    'authenticity_token' => $csrf_token
);

// Make POST request to the log in controller.
$ch = curl_init('https://diasp.org/users/sign_in');
curl_setopt($ch, CURLOPT_POSTFIELDS, $params);
curl_setopt($ch, CURLOPT_HEADER, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
// In order to work, the COOKIEFILE/JAR options must be used. Why?
//curl_setopt($ch, CURLOPT_COOKIEFILE, '/tmp/test_cookiejar');
//curl_setopt($ch, CURLOPT_COOKIEJAR, '/tmp/test_cookiejar');
$resp = curl_exec($ch);
curl_close($resp);

$cookies = readCookie($resp);

// Even if the login is successful, this fails if and only if no COOKIEFILE/JAR is specified.
// Why?
$ch = curl_init('https://diasp.org/stream');
curl_setopt($ch, CURLOPT_COOKIE, $cookies);
curl_setopt($ch, CURLOPT_HEADER, true);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
// If I use COOKIEFILE here, the request works. What is this line doing that CURLOPT_COOKIE is not?
//curl_setopt($ch, CURLOPT_COOKIEFILE, '/tmp/test_cookiejar');
$resp = curl_exec($ch);
curl_close($ch);

var_dump($resp);

SUMMARY

I am making very simple, step-by-step, procedural calls to a web server. These requests are being made one after the other, and the resulting output (of the entire HTTP conversation, including headers), is saved in a variable, which is then read and the values of the cookies are parsed from the Set-Cookie HTTP header lines. However, these values are never the same as the values that libcurl writes to the COOKIEFILE if those lines are uncommented.

What am I doing wrong with CURLOPT_COOKIE or what am I not doing with it that the CURLOPT_COOKIEFILE and CURLOPT_COOKIEJAR options are doing? Is it encoded or decoded in some reversible way? Thanks in advance.

You probably did not notice the difference between CURLOPT_COOKIE and CURLOPT_COOKIELIST/FILE/JAR. The both handle cookies but, CURLOPT_COOKIE does not store the cookies you set this time in the memory, or store them in the cookie file specified by CURLOPT_COOKIEJAR; instread, CURLOPT_COOKIELIST does.

There is a mechanism called cookie engine in libcurl. It is triggered enabled when you set any one of CURLOPT_COOKIELIST/FILE/JAR, libcurl takes care of sending/parsing/reading/storing cookies in all subsequent session.

CURLOPT_COOKIE is just a quick hack way to set a extra cookie for one go.