Protecting content within a page.

Hi All,
I'll ask here before I go off bothering other folks ...

First, some setup information. Site in PHP, and typical page will look like this:
<?php
some variables set here
?>

<? include the header file ?> (that contains the left side, and top of the page)

The contents of the file here. You know, click this link, click that link.

<? include the footer file ? (That contains the bottom of the page)

Now, what I want to do, hope to do, without redoing my entire site, is have content available for those who have logged in.

I've gotten it to work by placing
?php
session_start();
if($_SESSION[_amember_login]) {
$ismember = "1"; }
else { $ismember = "0"; }
?>

At the top of the page (before the header include).

However, if I place that same code in the header file, I get the error message "Warning: Cannot send session cache limiter - headers already sent"


Keep in mind I know enough PHP to really mess things up. (Read: I don't know if there is a PHP limitation that prevents me from doing this, that I don't know about.)

I hope that is enough information. If more is needed, please let me know.

So I guess the next obvious question is, do I need the session_start(); in there in order to make that work? (I could go try that out on my own, huh?)