Implementing the Solution

Step 1: Create File Structure

First, let's create our folder structure:

product_inheritance/
├── classes/
│   ├── Product.php
│   ├── PhysicalProduct.php
│   └── DigitalProduct.php
└── index.php
            

Step 2: Define the Base Product Class

Let's start by defining our base Product class from the previous homework:

File: classes/Product.php

<?php
/**
 * Base Product class
 * 
 * This class represents a generic product with basic properties and methods.
 */
class Product {
    // Protected properties (accessible to child classes)
    protected $name;
    protected $price;
    protected $description;
    
    /**
     * Constructor
     * 
     * @param string $name The name of the product
     * @param float $price The price of the product
     * @param string $description The product description
     */
    public function __construct($name, $price, $description = '') {
        $this->name = $name;
        $this->price = $price;
        $this->description = $description;
    }
    
    /**
     * Get the product name
     * 
     * @return string
     */
    public function getName() {
        return $this->name;
    }
    
    /**
     * Get the product price
     * 
     * @return float
     */
    public function getPrice() {
        return $this->price;
    }
    
    /**
     * Get the product description
     * 
     * @return string
     */
    public function getDescription() {
        return $this->description;
    }
    
    /**
     * Get product details as an array
     * 
     * @return array
     */
    public function getDetails() {
        return [
            'name' => $this->name,
            'price' => $this->price,
            'description' => $this->description
        ];
    }
    
    /**
     * Calculate product tax
     * 
     * @param float $rate The tax rate as a decimal (e.g., 0.1 for 10%)
     * @return float The calculated tax amount
     */
    public function calculateTax($rate = 0.1) {
        return $this->price * $rate;
    }
}
            

Step 3: Create the PhysicalProduct Class

Now, let's create our first child class for physical products:

File: classes/PhysicalProduct.php

<?php
// Include the parent class
require_once 'Product.php';

/**
 * PhysicalProduct class
 * 
 * This class extends the base Product class to represent
 * physical products with weight, dimensions, and shipping costs.
 */
class PhysicalProduct extends Product {
    // Additional properties specific to physical products
    protected $weight;
    protected $dimensions;
    
    /**
     * Constructor
     * 
     * @param string $name The name of the product
     * @param float $price The price of the product
     * @param string $description The product description
     * @param float $weight The weight in kilograms
     * @param array $dimensions The dimensions as [length, width, height]
     */
    public function __construct($name, $price, $description = '', $weight = 0, $dimensions = []) {
        // Call the parent constructor first
        parent::__construct($name, $price, $description);
        
        // Initialize our own properties
        $this->weight = $weight;
        $this->dimensions = $dimensions;
    }
    
    /**
     * Get the product weight
     * 
     * @return float
     */
    public function getWeight() {
        return $this->weight;
    }
    
    /**
     * Get the product dimensions
     * 
     * @return array
     */
    public function getDimensions() {
        return $this->dimensions;
    }
    
    /**
     * Calculate shipping cost based on weight and distance
     * 
     * @param float $distance The shipping distance in miles
     * @return float The calculated shipping cost
     */
    public function calculateShippingCost($distance) {
        // Basic formula: $5 base cost + $2 per kg + $0.10 per mile
        return 5 + ($this->weight * 2) + ($distance * 0.1);
    }
    
    /**
     * Get detailed product information
     * 
     * Override the parent method to include physical product details
     * 
     * @return array
     */
    public function getDetails() {
        // Get the basic details from the parent method
        $details = parent::getDetails();
        
        // Add our specific details
        $details['weight'] = $this->weight;
        $details['dimensions'] = $this->dimensions;
        $details['type'] = 'Physical Product';
        
        return $details;
    }
    
    /**
     * Calculate tax for physical products
     * 
     * May have different tax rules than other product types
     * 
     * @param float $rate The tax rate as a decimal
     * @return float The calculated tax amount
     */
    public function calculateTax($rate = 0.1) {
        // For example, physical products might have a different tax calculation
        // Here we're applying the tax rate to the price as in the parent class
        return parent::calculateTax($rate);
        
        // In a real application, you might have different rules, like:
        // return $this->price * $rate * (1 - $this->getDiscountMultiplier());
    }
}
            

Step 4: Create the DigitalProduct Class

Now, let's create our second child class for digital products:

File: classes/DigitalProduct.php

<?php
// Include the parent class
require_once 'Product.php';

/**
 * DigitalProduct class
 * 
 * This class extends the base Product class to represent
 * digital products with download URL and file size.
 */
class DigitalProduct extends Product {
    // Additional properties specific to digital products
    protected $downloadUrl;
    protected $fileSize; // In MB
    
    /**
     * Constructor
     * 
     * @param string $name The name of the product
     * @param float $price The price of the product
     * @param string $description The product description
     * @param string $downloadUrl The product download URL
     * @param float $fileSize The file size in MB
     */
    public function __construct($name, $price, $description = '', $downloadUrl = '', $fileSize = 0) {
        // Call the parent constructor first
        parent::__construct($name, $price, $description);
        
        // Initialize our own properties
        $this->downloadUrl = $downloadUrl;
        $this->fileSize = $fileSize;
    }
    
    /**
     * Get the product download URL
     * 
     * @return string
     */
    public function getDownloadUrl() {
        return $this->downloadUrl;
    }
    
    /**
     * Get the product file size
     * 
     * @return float
     */
    public function getFileSize() {
        return $this->fileSize;
    }
    
    /**
     * Generate a time-limited download link
     * 
     * @param int $validForHours Number of hours the link is valid
     * @return string The generated download link
     */
    public function generateDownloadLink($validForHours = 24) {
        // Generate a temporary token
        $token = md5(uniqid(rand(), true));
        
        // Create an expiry timestamp
        $expires = time() + ($validForHours * 3600);
        
        // In a real app, you'd store this token and expiry in a database
        
        // Return a URL with the token and expiry
        return $this->downloadUrl . '?token=' . $token . '&expires=' . $expires;
    }
    
    /**
     * Get detailed product information
     * 
     * Override the parent method to include digital product details
     * 
     * @return array
     */
    public function getDetails() {
        // Get the basic details from the parent method
        $details = parent::getDetails();
        
        // Add our specific details
        $details['download_url'] = $this->downloadUrl;
        $details['file_size'] = $this->fileSize;
        $details['type'] = 'Digital Product';
        
        return $details;
    }
    
    /**
     * Calculate tax for digital products
     * 
     * Digital products often have different tax rates
     * 
     * @param float $rate The tax rate as a decimal
     * @return float The calculated tax amount
     */
    public function calculateTax($rate = 0.05) {
        // Digital products often have reduced tax rates
        // Note we're using 0.05 (5%) as the default instead of 0.1 (10%)
        return $this->price * $rate;
    }
}
            

Step 5: Create a Test Script

Finally, let's create a test script to demonstrate our inheritance hierarchy:

File: index.php

<?php
// Include our class files
require_once 'classes/Product.php';
require_once 'classes/PhysicalProduct.php';
require_once 'classes/DigitalProduct.php';

// Create an instance of each class
$product = new Product('Generic Product', 19.99, 'A standard product');
$physicalProduct = new PhysicalProduct('Coffee Mug', 12.99, 'A ceramic coffee mug', 0.5, [10, 8, 12]);
$digitalProduct = new DigitalProduct('E-book: PHP Mastery', 24.99, 'Learn PHP from beginner to advanced', 'https://example.com/downloads/php-mastery.pdf', 15.5);

// Display product information
echo '<h2>Base Product:</h2>';
echo 'Name: ' . $product->getName() . '<br>';
echo 'Price: $' . number_format($product->getPrice(), 2) . '<br>';
echo 'Description: ' . $product->getDescription() . '<br>';
echo 'Tax (10%): $' . number_format($product->calculateTax(), 2) . '<br>';

echo '<h2>Physical Product:</h2>';
echo 'Name: ' . $physicalProduct->getName() . '<br>';
echo 'Price: $' . number_format($physicalProduct->getPrice(), 2) . '<br>';
echo 'Description: ' . $physicalProduct->getDescription() . '<br>';
echo 'Weight: ' . $physicalProduct->getWeight() . ' kg<br>';
echo 'Dimensions: ' . implode(' x ', $physicalProduct->getDimensions()) . ' cm<br>';
echo 'Tax (10%): $' . number_format($physicalProduct->calculateTax(), 2) . '<br>';
echo 'Shipping Cost (100 miles): $' . number_format($physicalProduct->calculateShippingCost(100), 2) . '<br>';

echo '<h2>Digital Product:</h2>';
echo 'Name: ' . $digitalProduct->getName() . '<br>';
echo 'Price: $' . number_format($digitalProduct->getPrice(), 2) . '<br>';
echo 'Description: ' . $digitalProduct->getDescription() . '<br>';
echo 'File Size: ' . $digitalProduct->getFileSize() . ' MB<br>';
echo 'Download URL: ' . $digitalProduct->getDownloadUrl() . '<br>';
echo 'Tax (5%): $' . number_format($digitalProduct->calculateTax(), 2) . '<br>';
echo 'Download Link: ' . $digitalProduct->generateDownloadLink() . '<br>';

// Demonstrate polymorphism by processing an array of different product types
echo '<h2>Product Details (Demonstrating Polymorphism):</h2>';
$products = [$product, $physicalProduct, $digitalProduct];

foreach ($products as $index => $prod) {
    echo '<h3>Product ' . ($index + 1) . ':</h3>';
    echo '<pre>';
    print_r($prod->getDetails());
    echo '</pre>';
}