PHP inside Javascript

Hi all, I would like to put some php inside javascript. I have done this before, but this time it's slightly different. The problem I'm having is that the PHP is being executed, despite the fact that it's inside a javascript function that hasn't been called yet. This is what I want to do: ------------------------ <BODY> <SCRIPT language="javascript"> <!-- function NextPage() { <?php if ($username == "") { $error = "You must enter a username"; } else if ($address == "") { $error = "You must enter an address"; } else { ?> document.frmDetails.submit(); <?php } ?> } //--> </SCRIPT> <FORM name=frmDetails> <INPUT type=submit name=btnNext id=btnNext value="Next>>" onclick="NextPage()"> </FORM> </BODY> ------------------------------ No matter what I do, the php is being executed and $error always has a value. Is there a way I can do PHP validation inside the javascript function and it not be executed? Thanks, Sam