Description:
------------
A couple things missing from the documentation on curl_getinfo:

1. (as mentioned in the comment http://php.net/function.curl-getinfo#83898)
To be able to use $opt=CURLINFO_HEADER_OUT you have to had curl_setopt'd CURLINFO_HEADER_OUT=true first.

2. On that subject,
If that option is set then (and only then) the associative array returned ($opt=0) will include a request_header item.

3. Also regarding that array,
It includes an unmentioned "certinfo" subarray. I'm not sure what this is for.

Test script:
---------------
$curl = curl_init("https://www.google.com");
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false); // if needed
curl_exec($curl);
print_r(curl_getinfo($curl)); // with certinfo, without request_header

curl_setopt($curl, CURLINFO_HEADER_OUT, true);
curl_exec($curl);
print_r(curl_getinfo($curl)); // with certinfo, with request_header

Expected result:
----------------
Tomorrow's lottery numbers

Actual result:
--------------
Array
(
...
    [certinfo] => Array
        (
        )
)

Array
(
...
    [certinfo] => Array
        (
        )

    [request_header] => GET / HTTP/1.1
Host: www.google.com
Accept: */*
)