I've commented out this block since main.php has to exist, as your comments say. We won't be checking if it exists.
// See if $content_file exists. If it does not, redirect to the homepage.
if (!file_exists($content_file)) {
//if I comment out this header line then the error goes away
//but I see only the header and not anything else
header("Location: {$_SERVER["PHP_SELF"]}");
exit();
}
*/
?>
<?=$title ?>