header function problems

[polarbear66]

I am having trouble redirecting to a page after I check to see if a variable is empty or not.

Here is my code:

<?php
session_start();
if ($sesCustomerID=="") {
header("Location:http://localhost/login.php");
exit;
}
?>

I get errors: Warning: Undefined variable: sesCustomerID in c:\inetpub\wwwroot\default1.php on line 5

Warning: Cannot add header information - headers already sent by (output started at c:\inetpub\wwwroot\default1.php:5) in c:\inetpub\wwwroot\default1.php on line 6

$sesCustomerID is a session variable I created on the page before. If the variable exists, then it works fine, the rest of the page loads correctly. If it doesn't exist, I need the user to be redirected to a login page. Please Help!

Thanks,

Joe

[junebug]

check if the variable is set instead

if(isset($sesCustomerID)){
header();
}

[polarbear66]

That didn't work either. I also tried:

if (!$sesCustomerID) {
header("Location:http://localhost/login.php");
}

I have looked everywhere and it seems like I am doing it correctly but for some reason its not working.