/// starting the session (first file - f.e. login.php)

setcookie('cookie_name',
          'cookie_name',
          time() + $session_length * 60);

session_name('session_cookie_name');
session_start();
$_SESSION['user_id'] = $some_id;

/// ending the session (second file - f.e. logout.php)

session_name('session_cookie_name');
session_start();
$_SESSION = array();
setcookie('session_cookie_name',
          'session_cookie_name',
          time() - 36000);
setcookie('cookie_name',
          'cookie_name',
          time() - 36000);
session_unset();
session_destroy();