form validation

I have a form which when you type a domain name into it ,it searches a database to see if that name is available,if it is it goes to a another form which they must fill out with their name and address etc,which is then mailed to me,i want to use php to validate the form but i 'm not too sure how to.,basically i want to check if they have filled in the requried fields and if not send them back to the page and place a graphic beside each of the fields not correctly filled in ,on my confirmation page which lists all the details they have supplied i have if (($first_name == "") && ($last_name == "") && ($address1 == "") && ($address2 == "") && ($town == "") && ($phone == "") && ($email == "") && ($paymentmethod == "")) { header("Location: http://wisemonkeydesign.co.uk/domain.php"); exit; but it doesnt work, I hope someone can help me solve these 2 problems Thanks in advance George