Why does XAMPP Sessions only work with autostart set to 1

Question

I can only get XAMPP to work with PHP Sessions whenever the php.ini setting auto_start is set to 1. Does anyone know why this is?

; Initialize session on request startup.
; https://php.net/session.auto-start
session.auto_start=1

At the start of all my php pages I call session_start() and initialise the variables:

if (!isset($_SESSION["basket"])) {
        echo "Setting up array";
        $_SESSION["basket"] = array();
    }