Newbie ? re: form validation

I've browsed around so much for answers to this question that now I'm completely confused. I have a couple simple forms with basic contact info and a script for inserting the values in a mySQL database. But now I want to include some basic validation, which I assume should go in the PHP script. I'd like to made sure that 1) the e-mail address is valid 2) the 2-character state code is entered correctly 3) a userid is unique I think I've found some script for the e-mail validation, but is there a built in php function for this, and if so, which is best? I don't have any idea how to do the state codes. Do I set up a database table with state abbreviations and then somehow check against it? How? Any advice about this subject would be greatly appreciated. Vicki