Check if Email Username Exists with CodeIgniter and jQuery Validation Plugin

A common feature of most new user registration forms is a message telling you whether your chosen username or e-mail is already taken. The check is done through an AJAX request. Using the jQuery Validation plugin, the remote option can be used to request a URL that returns JSON data. Using CodeIgniter, this URL is a new function in the controller used for registration. In this example, the register_email_exists() function returns true or false, after checking the entered form value against the database. See the following example code for the basic PHP and JavaScript you'll need to make this work.