Passing values from ajax to php class functions

Q. Is there a way to pass values from ajax to a certain php class having functions? Let's say validating a username on the registration form whether the user exist or not. This is a simple form that will accept username and has a span tag to display the message. <form action="" method="POST"> <input type="text" name="username"><span class="check"></span> <input type="submit" name="signup"> </form> And for the php class: <?php class User { function isUserExist($username) { $query = mysql_query("SELECT username FROM users WHERE username='$username'"); $result = mysql_num_rows($query); return ($result !== 0 ? true : false); } } ?> It is initialized on the php class that established connection to the database. So calling to the php page will become like this: $user->isUserExist($_POST['username']);. So is it possible to pass values from the form to ajax and send it to the php class function?