PHP : Sign Up Form animating

i want to do a sign up form who开发者_JS百科 will instantly check the answer and if the form is valid, a green circle appear beside the form.

You need to do this with Javascript and CSS:

Try something like this: http://bassistance.de/jquery-plugins/jquery-plugin-validation/

if you only want to check for the correct length and characters used, javascript will do. If you also want to perform checks like whether the username or email already exist, things get more complicated. You could achieve that with AJAX http://en.wikipedia.org/wiki/Ajax_(programming)

but be aware of webbots, because if you provide such functionality they can expose the existing users. Thats why login forms often answer "Username or password incorrect" although the user exists.