E-Commerce Logic 🛒

Model shopping carts with coupons, build order processing pipelines with inventory checks, and generate professional invoices.

What You'll Learn in This Lesson

• Build a shopping cart with add, remove, and quantity updates
• Apply coupon codes with percentage discounts
• Calculate subtotals, tax, and grand totals
• Process orders through a status lifecycle
• Generate formatted invoices and track order timelines

Note: PHP runs on a server. Our examples simulate PHP logic using JavaScript. To run real PHP, install PHP.

🎯 Real-World Analogy: An e-commerce system is like a physical retail store. The shopping cart is the trolley customers push around. Adding items checks inventory (warehouse). At checkout, the cashier (order processor) applies coupons, calculates tax, takes payment, and prints a receipt (invoice). The order then moves through fulfillment: picking → packing → shipping → delivered.

Shopping Cart System

A cart needs to handle adding items (merging duplicates), updating quantities, removing items, and applying discount coupons. Store prices in cents (integers) to avoid floating-point rounding errors — $29.99 becomes 2999. Calculate tax only on the discounted subtotal.

Try It: Shopping Cart

Build a cart with items, coupons, tax calculation, and totals

Try it Yourself »

JavaScript

// Shopping Cart System in PHP
console.log("=== Building a Shopping Cart ===");
console.log();

class ShoppingCart {
  constructor(userId) {
    this.userId = userId;
    this.items = [];
    this.coupon = null;
  }

  addItem(product, quantity) {
    let existing = this.items.find(i => i.product.id === product.id);
    if (existing) {
      existing.quantity += quantity;
      console.log("  📦 Updated: " + product.name + " × " + existing.quantity);
    } else {
      this.items.push({ product,
...

Orders & Invoices

Orders have a lifecycle: pending → paid → processing → shipped → delivered. Each status change creates a timeline entry for customer tracking. Inventory checks prevent overselling, and invoices provide a permanent record of the transaction for accounting and tax purposes.

Try It: Order Processing

Check inventory, process orders through lifecycle, and generate invoices

Try it Yourself »

JavaScript

// Order Processing & Invoice Generation
console.log("=== Order Lifecycle ===");
console.log();

class Order {
  constructor(cart, customer, shippingAddress) {
    this.id = "ORD-" + Date.now().toString(36).toUpperCase();
    this.items = cart.items.map(i => ({
      productId: i.product.id,
      name: i.product.name,
      price: i.product.price,
      quantity: i.quantity,
      lineTotal: i.product.price * i.quantity,
    }));
    this.customer = customer;
    this.shipping = shippingAddress
...

⚠️ Common Mistakes

⚠️

Using floats for money — 0.1 + 0.2 !== 0.3 in JavaScript and PHP. Store prices as integers (cents) and divide by 100 only for display.

⚠️

Not locking inventory — Two users can buy the last item simultaneously. Use database transactions with SELECT ... FOR UPDATE to prevent race conditions.

💡

Pro Tip: Store cart in the database (not just session) so it persists across devices. When a user logs in on their phone, their desktop cart appears.

📋 Quick Reference — E-Commerce

Concept	Description
Cart	Temporary collection of items before checkout
Order	Confirmed purchase with payment and shipping
SKU	Stock Keeping Unit — unique product identifier
Idempotent Payment	Ensure charge happens exactly once
Cents (integers)	Store money as integers to avoid float errors

🎉 Lesson Complete!

You can now build e-commerce systems! Next, learn server-side rendering with template engines like Twig.