Redirecting in a PHP script

I need to "goto" different PHP pages in my web site depending on what happens within some PHP code. For example, if the user is not logged in when he goes to a page, I want to send him to a LOGIN page. I've have everything working fine, using the following Javascript code:

            $location = 'login.php';
                echo "<script language='javascript'>\n";
                echo "document.location.href = '" . $location . "';\n";
                echo "</script>\n";

I also played around with using the header("location: ...") function. I understand that the header() function must be issued before any HMTL is output. But I'm not sure about the Javascript code. In every instance in my code, I use the Javascript before any HTML - this type of action normally occurs in PHP code called via a form POST. I presume that the Javascript code really does the same as the PHP stuff, and thus must obey the same rules, but I'm not sure. Comments? Larry Bradley Orleans (Ottawa), Ontario, CANADA