General Support: Warning: Cannot modify header information

I'm getting these warning all over the place on plogger. Some completely stop plogger, others are displayed at the top of a page, with plogger loaded underneath.

Warning: Cannot modify header information - headers already sent by (output started at /home/content/j/u/s/justin1186/html/plogger/plog-globals.php:92) in /home/content/j/u/s/justin1186/html/plogger/admin/plog-admin.php on line 6

Another issue with this warning stuff: After I've logged into plogger admin this statement is failing:
1. Because of the header
2. I've logged in...why isn't the session set???

if (!isset($_SESSION["plogger_logged_in"])){

header("Location: index.php");

exit;

}