Forward singular posts to page

I’m using the following code as I don’t want people to view singular custom posts. If they try to visit one, I want them to get forwarded to a page:

<?php

	// forward necessary single custom posts
	$forward = 0;

	if(is_singular('external-link')) {
		$location = get_permalink(12);
		$forward = 1;
	}
	if ($forward == 1) {
		header('Location: ' . $location);
	}
	exit;

?>

However even with error reporting turned on I just get a blank white screen.

I normally use the following script on most websites I make:

<?php
	// forward necessary single custom posts

	if(is_singular('external-link')) {
		$location = get_permalink(12);
		$forward = 1;
	}
	if ($forward) {
		header('Location: ' . $location);
	}
	exit;
?>

However on this site it is just giving me this error:

Notice: Undefined variable: forward in /home/devspare/public_html/wp-content/themes/sparetimetocash/header.php on line 8

Can anyone see any issues?? It seems like if I resolve the error messages all I get is a white screen of death