Implement validation using ajax along with php

[xx-man]

I am building a website using php+mysql. I have a registration page and its already implemented in php. Now i would like to make the page more dynamic and interactive using ajax.That is i would like to have a feature in which when user moves to next tab it should display message in previous field saying right/wrong depending on user entered data. Can anyone let me know is it possible to add this feature to already php page or should i start from scratch. Please let me know if any good sources for learning ajax