There is a similar posting but they don't have id's so that was the topic title 'Chap 12 - 12.6 Example' which doesn't look like it was resolved. Due to the advances in web browsers I can't review the cookies as per the example K on page 387.
First of all, on script 12.7, there seems to be a typo on line 3: <head"> (the double quotation mark).
When I first load index.php the header.html displays Logout which seems strange as I haven't logged in yet. Selecting Logout takes me back to index.php and the header.html is still displaying Logout.
When I load login.php directly I can login and the browser displays 'first_name' and the logout button below.
However, when logged in, the header is displaying Login.
I can select logout below 'first_name' and am logged out with the logout.php script but the header still displays Login. I have tested in Firefox and Chrome with the same results. I have checked the scripts character ny character.
Apache Version: 2.4.23
PHP Version: 5.6.25
Firefox: 58.0.2 (64-bit)
Chrome: Version 64.0.3282.186 (Official Build) (64-bit)