This tutorial will show you how you can check username availability using Codeigniter, AJAX and MySQL. Sometimes we need to check username availability instantly before a user presses the submit button after filling a long-sized signup form. In this case we can use AJAX technique with any server side technologies like PHP, Codeigniter, Servlet, Struts, JSF, Spring etc. for checking whether the input username is avaialable for creating new user account or it has already been taken by someone else. So if we give instant result to the user for username availability then sometimes it makes more sensible than while pressing the submit button and goes top of the signup form to rectify the username input field if input username is not available. So after finishing this example “username check availability using Codeigniter, AJAX and MySQL” you will be able to apply the same logic to any server side technologies.