Ajax combined with PHP or any other server side script can be used to develop powerful scripts. The main advantage of using Ajax is better user experience as page need not refresh again and again to interact with database or any other facility.
Recently while testing on form validation we found it is highly useful and not so difficult to implement. You can check this demo on form validation
Further we have extended and added one signup form and kept a demo here.
let us know how do you like this ... any suggestion for improvement or modification will be highly appreciated.