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Documentation Index

Fetch the complete documentation index at: https://mintlify.com/wreyesus/Sencillo_Carrito_de_Compras_en_PHP/llms.txt

Use this file to discover all available pages before exploring further.

This project relies entirely on PHP’s native session mechanism — no database writes happen for cart state until the order is placed. Every page in the store calls session_start() at the top, and two logical groups of session keys drive the entire experience: one that identifies the authenticated user and one that holds the live contents of the shopping cart.

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Session keys

The table below covers every $_SESSION key used across the project. All three are set and read without any intermediate caching layer.
KeyTypeDescription
$_SESSION['usuario']stringThe logged-in user’s nickname, exactly as stored in the usuario table. Set on successful login; checked on every page that requires authentication.
$_SESSION['idusuario']intThe user’s primary key (idusuario) from the usuario table. Set alongside 'usuario' on login so that pages can reference the user’s database record without an extra query.
$_SESSION['carrito']arrayAn array of cart-item arrays (see structure below). Written whenever an item is added, updated, or removed. Persists across all pages until the session is destroyed.

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Cart session structure

Each product the shopper adds to the cart is appended to $_SESSION['carrito'] as an associative array with three keys. The outer array is numerically indexed and grows with each unique product that is added. 
// Each element added to the cart
$_SESSION['carrito'][] = [
    'nombre'   => 'Core i3 2100 3.01Ghz',  // product name
    'precio'   => '100.00',                  // unit price
    'cantidad' => 2,                         // quantity
];
When a shopper removes an item from the cart (carrito.php receives $_POST['id2']), the corresponding element is set to NULL rather than being spliced out of the array: 
// Remove item at index $id
$compras[$id] = NULL;
$_SESSION['carrito'] = $compras;
This means array indices are stable — the position of every remaining item in the array never shifts. All downstream pages that loop over the cart check if ($item != NULL) before processing each element.

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Session lifecycle

1

Start session

Every PHP page in the project opens with session_start(). This either resumes an existing session identified by the browser’s cookie or creates a new empty session.
2

Login

When a user submits valid credentials, index.php or carrito.php runs a SELECT against the usuario table. On success it sets both session keys and redirects:
$_SESSION['idusuario'] = $rowsresult['idusuario'];
$_SESSION['usuario']   = $nickname;
header("location:carrito.php?login=true");
3

Add to cart

carrito.php reads the incoming POST fields (nombre, precio, cantidad) and either appends a new item or increments the quantity if the same product name already exists in $_SESSION['carrito']:
if ($duplicado != -1) {
    $cantidad_nueva = $compras[$duplicado]['cantidad'] + $cantidad;
    $compras[$duplicado] = [
        'nombre'   => $nombre,
        'precio'   => $precio,
        'cantidad' => $cantidad_nueva,
    ];
} else {
    $compras[] = ['nombre' => $nombre, 'precio' => $precio, 'cantidad' => $cantidad];
}
$_SESSION['carrito'] = $compras;
4

Checkout

resumenc_compra.php reads $_SESSION['carrito'] to display the order summary. final.php reads the same key to build the order confirmation email body and to calculate the PayPal total that is passed to the payment form.
5

Logout

cerrar_sesion.php calls session_destroy() and unsets the session superglobal, clearing both the user identity and the cart in a single operation.

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Guest vs. authenticated cart

Guests can browse products and even trigger the add-to-cart POST from any product page without being logged in. The cart data is written to $_SESSION['carrito'] regardless of authentication state. However, carrito.php checks for a logged-in user before rendering the cart table — if no session exists it shows inline login and registration forms instead: 
if (!isset($_SESSION['usuario'])) {
    // show login/register forms inline
}
This means a guest can add items, then log in on the cart page, and their cart will already be populated — the items added before login are preserved in the session.

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Gotchas

Because items are NULLed rather than spliced, the numeric index of each item never changes during a session. The update form on carrito.php passes the array index as a hidden id field. If item 0 is removed (set to NULL) and item 2 is still present, the update POST for item 2 still uses id=2 — which is correct. The pattern can be confusing when iterating the array: always loop from 0 to count($compras) - 1 and skip NULL entries.
final.php sends the order confirmation email and redirects the shopper to PayPal, but it does not call session_destroy() or unset $_SESSION['carrito']. If the user presses the browser back button or navigates back to carrito.php after completing payment, the cart will still contain the items from the previous order. In a production application, the cart should be cleared after a successful order is submitted.
Every PHP page in the project calls session_start() at the very top of the file, before any HTML output. If a page accidentally emits whitespace or a byte-order mark before session_start() is reached, PHP will emit a Cannot send session cookie - headers already sent warning and the session will not be created. Keep the opening <?php tag on the very first byte of every file and do not add blank lines before it.

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