AJAX form validation allows checking user input without reloading the page.
This improves user experience and prevents invalid data submission.
JavaScript sends input value to PHP for validation.
PHP checks whether the input already exists in database.
Validation result is returned instantly to the user.
AJAX validation reduces server load and improves form usability.
This technique is commonly used in registration forms and login systems.
After completing this lesson, you can build real-time validation using AJAX.