Multiple Operators in a Real Application

<?php
// Shopping cart system example
class ShoppingCart {
    private $items = [];
    private $total = 0;
    private $tax_rate = 0.08; // 8% tax
    
    // Add item to cart
    public function addItem($product_id, $name, $price, $quantity = 1) {
        // If product already exists, update quantity
        if (isset($this->items[$product_id])) {
            $this->items[$product_id]['quantity'] += $quantity;
        } else {
            // Otherwise add new item
            $this->items[$product_id] = [
                'name' => $name,
                'price' => $price,
                'quantity' => $quantity
            ];
        }
        
        // Update total
        $this->total += $price * $quantity;
        
        return $this;
    }
    
    // Remove item from cart
    public function removeItem($product_id, $quantity = null) {
        if (!isset($this->items[$product_id])) {
            return $this;
        }
        
        // If quantity is null or >= current quantity, remove the entire item
        if ($quantity === null || $quantity >= $this->items[$product_id]['quantity']) {
            $this->total -= $this->items[$product_id]['price'] * $this->items[$product_id]['quantity'];
            unset($this->items[$product_id]);
        } else {
            // Otherwise reduce the quantity
            $this->items[$product_id]['quantity'] -= $quantity;
            $this->total -= $this->items[$product_id]['price'] * $quantity;
        }
        
        return $this;
    }
    
    // Apply discount percentage
    public function applyDiscount($percentage) {
        if ($percentage <= 0 || $percentage >= 100) {
            return $this;
        }
        
        $discount_factor = $percentage / 100;
        $this->total *= (1 - $discount_factor);
        
        return $this;
    }
    
    // Get cart summary
    public function getSummary() {
        $subtotal = $this->total;
        $tax = $subtotal * $this->tax_rate;
        $total = $subtotal + $tax;
        
        return [
            'items' => $this->items,
            'subtotal' => $subtotal,
            'tax' => $tax,
            'total' => $total
        ];
    }
    
    // Display cart
    public function display() {
        $summary = $this->getSummary();
        
        $output = "<h3>Shopping Cart</h3>";
        
        if (empty($this->items)) {
            $output .= "<p>Your cart is empty.</p>";
            return $output;
        }
        
        $output .= "<table border='1'>";
        $output .= "<tr><th>Product</th><th>Price</th><th>Quantity</th><th>Subtotal</th></tr>";
        
        foreach ($this->items as $id => $item) {
            $subtotal = $item['price'] * $item['quantity'];
            $output .= "<tr>";
            $output .= "<td>{$item['name']}</td>";
            $output .= "<td>$" . number_format($item['price'], 2) . "</td>";
            $output .= "<td>{$item['quantity']}</td>";
            $output .= "<td>$" . number_format($subtotal, 2) . "</td>";
            $output .= "</tr>";
        }
        
        $output .= "</table>";
        $output .= "<p>Subtotal: $" . number_format($summary['subtotal'], 2) . "</p>";
        $output .= "<p>Tax (8%): $" . number_format($summary['tax'], 2) . "</p>";
        $output .= "<p><strong>Total: $" . number_format($summary['total'], 2) . "</strong></p>";
        
        return $output;
    }
}

// Use the shopping cart
$cart = new ShoppingCart();

// Add items
$cart->addItem(101, 'Widget A', 19.99, 2)
     ->addItem(102, 'Widget B', 29.99)
     ->addItem(103, 'Widget C', 39.99, 3);

echo $cart->display();

// Apply discount
$cart->applyDiscount(10); // 10% discount
echo "<p>After 10% discount:</p>";
echo $cart->display();

// Remove an item
$cart->removeItem(101, 1); // Remove 1 Widget A
echo "<p>After removing one Widget A:</p>";
echo $cart->display();
?>