I have made a very basic registration page whereby hitting 'submit' will add the details into my Users table with no validation or anything.

What I want to do now is start setting the login/registration side of my site up so just a few questions I hope you can answer:

1. When I fill in my registration form I would like to perform validation and then authentication. I understand that I can pass the form values to authentication.php (or whichever file) and can do the checks there however is there a way I can do the validation first to make sure the form is filled in corectly AND maybe display a line on that page. Eg. if username is not filled in, it could just say 'please fill in a username' -- I'm thinking of using the 'or die' function and just making the user press back to refill the form, but is there a better way??

[I have a few more but I guess I should ask them when this one is done]