1. Change the file name for index.html to index.php
2. Add PHP code to index.php to display an error message named $error_message just below the page heading. Be sure to check that $error_message is not an empty value. Format it to be bold, red text.
3. Display error messages if the Product Description, List Price and Discount Percent fields are empty after the user submits the form. (1)
4. Display an error message if the List Price or Discount Percent data entered is not a number after the user submits the form.
5. Display an error message if the List Price or Discount Percent data entered is less than zero after the user submits the form.
6. If an error message is displayed, take the user back to the index.php page.
7. Add a sales tax calculation of 8% based on the discounted price. Then, display the sales tax rate and the calculated sales tax amount after the discounted price.