about forms with php

Hi, I want to learn if anyone tried to use both of javasccript and php together ?Because when I tried to use them like this: <form name="sign" method="POST" action="sign_up_.php" onSubmit="checkemail(email.value);return false" > and when I click submit.. javascript is working but it is not going to the sign_up_ page ......How can I manage it?.. thanks alot.. by the way , by using javascript , I am checking if the email adress are written correctly ( '@' ...).. thanks meltem