check the availability of username using php ajax code in a simpler way

[hostsoftware]

I have a registration form in which all the fields are validated using JavaScript. I need to check the username availability , that is, whether that name already exists in my database(MySql) or not. If it exists simply an alert box should be popped saying "Username already exists" and the user should not be able to proceed to fill other fields. I need a simpler solution for this without jquery usage. Is there any solution for this using PHP and ajax?