Warning: Cannot modify header information - headers already sent by (output started at /home/blocexco/public_html/homepage.php:73) in /home/blocexco/public_html/classes/mysql/mysql.security.php on line 99

This error is repeated a second time for mysql.security.php on line 100.

homepage:73

<div class="login">
<?php require_once 'login.php'; ?>
</div>

mysql.security.php: 99-100

      setcookie('username', "", time() - (60 * 60 * 24 * 365));
      setcookie('password', "", time() - (60 * 60 * 24 * 365));

I know this isn't a "BOM" issue as I've read about. There is output before and after my calls to header() and setcookie() functions - this is necessary since the homepage includes a php file which then injects the right login or logout form.

I've heard about using ob_start() at the beginning of content, but that's not a very specific instruction...I tried placing it at the beginning of homepage.php (just before the html tag) and that didn't fix anything.

I'm new to PHP (a few days in, and new to web-app dev in general). To be honest, it blows my mind that I can't just change which page I am on, via php without bending over backwards...