✏ EDIT: cart_add.php

<?php
// Adds an item to the cart stored in the session. If the item
// already exists in the cart, its quantity is incremented. The cart
// structure is an associative array keyed by item IDs.

require_once __DIR__ . '/bootstrap.php';

require_login();

$input = json_decode(file_get_contents('php://input'), true);
if (!$input) {
    $input = $_POST;
}

$itemId = isset($input['item_id']) ? (int)$input['item_id'] : 0;
$qty = isset($input['qty']) ? (int)$input['qty'] : 1;

if ($itemId <= 0) {
    respond(null, 'Invalid item ID');
}
if ($qty <= 0) {
    $qty = 1;
}

// Ensure the item exists and is active
try {
    $stmt = $pdo->prepare('SELECT id FROM items WHERE id = ? AND is_active = 1');
    $stmt->execute([$itemId]);
    if (!$stmt->fetch()) {
        respond(null, 'Item not found');
    }
    if (!isset($_SESSION['cart']) || !is_array($_SESSION['cart'])) {
        $_SESSION['cart'] = [];
    }
    if (!isset($_SESSION['cart'][$itemId])) {
        $_SESSION['cart'][$itemId] = 0;
    }
    $_SESSION['cart'][$itemId] += $qty;
    respond($_SESSION['cart']);
} catch (PDOException $e) {
    respond(null, 'Database error');
}

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