The actual headers that are set using the values described above are:

public:
    Expires: pageload + 3 hours
    Cache-Control: public, max-age=10800
    
private:
    Expires: Thu, 19 Nov 1981 08:52:00 GMT
    Cache-Control: private, max-age=10800, pre-check=10800
    
nocache:
    Expires: Thu, 19 Nov 1981 08:52:00 GMT
    Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
    Pragma: no-cache
    
private_no_expire:
    Cache-Control: private, max-age=10800, pre-check=10800
    

Regarding other settings mentioned by some, those just don't do anything. Check the source of PHP, in ext/session/session.c -- the above values are all that actually do anything. Other values, or an emtpy string, result in no cache-limiting headers being set at all.