Stuck on include() in a wordpress theme

Stuck on include() in a wordpress theme

I'm going to include() a php file in a wordpress theme file. The file is
header.php, and the function is like:
<?php
error_reporting(E_ALL);
$filename = ...; // file exists but is empty
if (file_exists(filename)){
echo 'Ok';
include($filename);
} ?>
"Ok" is printed in resulting html, but output stops immediately after. Am
I missing something about home themes work?
File permissions are ok.