CakePHP的session.php中的以下代码是做什么的？

Question

I am trying to understand a method in the CakePHP source code. Following is the code taken from session.php.

    function __startSession() {
        if (headers_sent()) {
            if (empty($_SESSION)) {
                $_SESSION = array();
            }
            return false;
        } elseif (!isset($_SESSION)) {
            session_cache_limiter ("must-revalidate");
            session_start();
            header ('P3P: CP="NOI ADM DEV PSAi COM NAV OUR OTRo STP IND DEM"');
            return true;
        } else {
            session_start();
            return true;
        }
    }

Why not just session_start? Why the checks for headers_sent and what is the meaning of header ('P3P: CP="NOI ADM DEV PSAi COM NAV OUR OTRo STP IND DEM"');? IN what situations will empty($_SESSION) or !isset($_SESSION) evaluate to true? An explanation of all the code is highly appreciated.

Answer 1

quick googling on the header provides some nice info.
It is for IE6, to fix its problem with third party cookies (in cases where you db data come from a different domain/IP than your webserver).
You can read more here