“Cannot start session when headers already sent” when attempting session

It turns out not all hosts treat sessions alike. I ended up scraping altogether the “session” code in functions.php previously referred to which was doing nothing.

Instead, I added session_start() at the top of every template adding or retrieving a value from a name/value hash in $_SESSION. So,

<?php 
    session_start();

    $_SESSION['order_id'] = $order_id;
    . . .
    $order_id = $_SESSION['order_id'];