Php session cart

General information for the website: generan cart
Kind of development: Customization of existing website
Description of requirements/features: Hello!

I need some help to build a shopping cart with session in a php custom eshop.
Requirements:
I have an index page with these products.
Product 1
Product 2
Product 3

When customer push these link, redirect him to cart page, where product id stored in a session and shows the cart total.
When customer push again a product from product page, the cart must stored in same session as quantity +1
After that i need the output stored in my database. If customer add to cart more than one different product, i need a new row on orders.
To avoid injections, i need to check first if product id is on database and the id is integer.

In attachements, i give you the structure of my database.
In my index page, i print all the products from all Companies (Company1,2,etc).

I tried to google that and i test this code that suits me. I think this is very old and i get a lot of errors.
https://jameshamilton.eu/programming/simple-php-shopping-cart-tutorial
You can use this if you know how to fix the code.

I think that someone who knows php very good must do it in some minutes.
Please dont make me any more expensive offers.
Extra notes: