PHP is not storing $_SESSION array values - php

Question

I'm new in php and right now I'm having a problem when storing data in the $_SESSION array. I have an index page that retrieves information from an XML file and displays each product from the XML file in the page each one with a button to add to a cart. When the button is pressed, I send an XMLHttpRequest with the id of the product I want to add to the $_SESSION array.
This is the main page:
function check_avi(id){
var xhttp = new XMLHttpRequest();
xhttp.onreadystatechange = function() {
if (this.readyState == 4 && this.status == 200) {
var available = this.responseText;
if (available == "True") {
alert("Add to the cart successfully.");
} else {
alert("Sorry, the car is not available now. Please try other cars.");
}
}
};
xhttp.open("GET", "add_delete_session.php?action=add&id="+id, true);
xhttp.send();
In the php I open the xml, retrieve the info and store it into an array, I then copy the information to the $_SESSION[#number of product]
What is happening is that after I add a product, it deletes in the session array the previous products, so each time I'm only having one product in the array and I want to store all of them to show them later in a shopping cart. The php should return a variable to the main page indicating the availability of the product but I'll do that later.
$arra is the array in where I'm storing the values obtained from the XML file corresponding to the product of the Id sent from the main page, like the price or name. $rf is the variable which count the amount of products stored and serve the same function as $_SESSION['g']
This is my PHP:
<?php
session_start();
$id = $_GET[id];
$action = $_GET[action];
if (empty($_SESSION['g'])) {
$_SESSION['g'] = 0;
}
switch($action){
case "add" :
$xmlDoc = new DOMDocument();
$xmlDoc = simplexml_load_file("cars.xml");
for ($e = 0; $e < 10; $e++) {
$cont = 0;
unset($arra);
foreach ($xmlDoc->carrito[$e]->children() as $child) {
$arra[$cont] = $child;
$cont = $cont + 1;
}
if ($arra[0] == $id) {
for ($f = 0; $f < 10; $f++) {
$rf = $_SESSION['g'];
$_SESSION[$rf][$f] = $arra[$f];
}
$_SESSION['g'] = $_SESSION['g'] + 1;
}
}
break;
case "delete":
break;
}
If anyone could help me I'll be so grateful.
Thanks

Answer 1

I think your in your code
1:
if (empty($_SESSION['g'])) {
$_SESSION['g'] = 0;
}
Actually here you haven't even defined the session at all and you are checking for its value if empty or not that's gonna stop execution of your code there itself, first you need to define the session variable and the check the value for it.
2:
$rf = $_SESSION['g'];
$_SESSION[$rf][$f] = $arra[$f];
Because it means that
$_SESSION[$_SESSION['g']][$f] = $arra[$f];
Which is a wrong approach.
Better way first you make all the simplifications and then assign it to the $_SESSION[] and echo it.