Cookie Value Megaupload.com How Do I Set A Cookie With A Variable As Its Value In Php?

How do I set a cookie with a variable as its value in php? - cookie value megaupload.com

I want to do something like this:

setcookie ( "login cookie", $ username)

The problem is, it seems, these are put into the headers before the HTML or PHP, and therefore not the variable $ username is set. Is there any way to $ users prefer bottom-up in all the cookies? Or what my other options? Thank you!