Updating URL for parsing XML in PHP - php

Question

I am trying to figure out how to update an URL that I am using to parse the XML (using simplexml_load_file()).
There is a button that has a $productid that is assigned to it by the user when they add their shortcode [show_cart_button productid=#].
//code for the button input that gets called
function print_add_cart_button($productid, $atts = array()) {
//some other form code for the button
$replacement .= '<input type="hidden" name"cart_product" value"' . $productid . '"/>';
return $replacement;
}
This is the function I am using to display and call the product information:
function show_shopping_cart_handler($atts) {
if (isset($_POST['cart_product'])) {
$id = "&PRODUCTID=" . $_POST['cart_product'];
// uses the input name from the button
}
$url = "https://secure.bmtmicro.com/cart?CID=2/WP" . $id;
$xml = simplexml_load_file($url) or die("error.");
foreach ($xml->producttable->row as $product) {
$output .= '<span>' . $product->productname . '</span>';
}
return $output;
}
The way the URL will work properly is to have a &PRODUCTID=(product#) added to the end of the URL. For example, if you click two different buttons, the URL should look like this: https://secure.bmtmicro.com/cart?CID=2/WP&PRODUCTID=1&PRODUCTID=2 (obviously 1 and 2 will need to display the buttons $productid number).
As you should be able to see from my code above, every time a button gets clicked, it just rewrites the URL and displays that one product, instead of adding a new one to the list. I tried to set the simplexml_load_file() into a foreach(), but that would not display any products when the button was clicked. Does anyone know a way I could get that URL to update/work properly?

Answer 1

Everytime
if (isset($_POST['cart_product'])) {
$id = "&PRODUCTID=" . $_POST['cart_product'];
// uses the input name from the button
}
Is called, it's overwriting $id. You need to get any existing values for the url.
You'll have to get all of the current values ($_GET['cart_productions']), throw them into an array, grab the new post, add it to the array and add all of them to them the url using http_build_query().