To validate the checkout form using Ajax and PHP on server side. User's input is validated on the fly. If error is found, appropriate information is displayed. All required fields should be checked for completion, and all data that are entered should be checked for 'correctness'. For example, buyer contact details (name, phone number, email address, etc.), delivery address (unit number/street name, city, postal code, etc.), delivery method selected and payment methods (credit card number, expiry date, card holder name, CVV, etc.) are fields expected in the checkout form to be validated. Regular expression is recommended (NOT compulsory) for pattern matching in PHP. You decide what validation is required and how the validation is performed. For instance, the required fields checked for completion the fields checked for a specific length, numeric data or a predetermined format the messages provided help the user fix the 'problem'