Using Sessions for High Traffic with AJAX

I still use CodeIgniter version 3.

I'm basing myself on this sentence from the session documentation (Session Library — CodeIgniter 3.1.13 documentation):
Long story short - call session_write_close() once you no longer need anything to do with session variables. 

The problem is that sometimes I use session at the beginning of the code, then halfway through the code.

In this case from what I understand the best thing would be for my code to be like this, correct?

PHP Code:

function setSession($index, $value) {

    if (session_status() === PHP_SESSION_NONE) {
        session_start();
    }

    $_SESSION[$index] = $value;

    session_write_close();
}

function getSession($index) {

    if (session_status() === PHP_SESSION_NONE) {
        session_start();
    }

    $value = FALSE;

    if (isset($_SESSION[$index]) {
        $value = $_SESSION[$index];
    }

    session_write_close();
}