PHP Shopping Cart using Sessions

What we are building

A shopping cart remembers what a user wants to buy as they browse. We will build a simple cart using the session, so it survives across pages without a database until checkout. It will add items, update quantities, show the total, and remove items.

Starting the cart

<?php
  session_start();
  if (!isset($_SESSION["cart"])) {
      $_SESSION["cart"] = [];   // empty cart
  }
?>

We start the session and make sure the cart array exists. Every cart page begins like this so the cart is always ready to use.

Add to cart

<?php
  $id = (int)$_GET["id"];   // product id

  if (isset($_SESSION["cart"][$id])) {
      $_SESSION["cart"][$id] += 1;   // already there: +1
  } else {
      $_SESSION["cart"][$id] = 1;    // new item
  }
  echo "Added to cart";
?>

If the product is already in the cart, we increase its quantity; otherwise we add it with quantity 1. Using the id as the key keeps one entry per product.

View the cart and total

<?php
  // pretend prices come from the database:
  $prices = [1 => 100, 2 => 250, 3 => 90];

  $total = 0;
  foreach ($_SESSION["cart"] as $id => $qty) {
      $line = $prices[$id] * $qty;
      $total += $line;
      echo "Product $id x $qty = $line\n";
  }
  echo "Total: $total";
?>

We loop the cart, look up each price from a trusted source, multiply by quantity, and add to the total. Never trust a price sent from the browser.

Remove an item

<?php
  $id = (int)$_GET["id"];
  unset($_SESSION["cart"][$id]);   // remove that product
  echo "Removed from cart";
?>

unset deletes that product from the cart array. To empty the whole cart, set $_SESSION["cart"] = [].

Common mistakes

• Trusting prices from the form. Always fetch the real price from the database by id.
• Not casting the id. Cast $_GET["id"] to (int) for safety.
• Duplicate entries. Key the cart by product id and increase quantity instead of adding again.
• Forgetting session_start(). Every cart page needs it to read the cart.

Summary

• A shopping cart lives in the session while the user browses.
• Store it as an array keyed by product id, each holding a quantity.
• Adding an existing product increases its quantity.
• Calculate the total from trusted database prices, not the browser.
• Use unset to remove an item; clear the array to empty the cart.

Next, save the final order with PHP CRUD, or manage the session in sessions and cookies.