Auto_prepend_file Help

I’ve been trying to force headers on one of my subdomains recently, using .htaccess and a header.html file, except I’ve been having a bit of trouble.
I was wondering whether someone could point me in the direction of the correct syntax to use in my .htaccess file?
I’ve been trying something like:

AddHandler application/x-httpd-php .htm .html
php_value auto_prepend_file “header.php”

And lots of variations on that but I can’t get it to work.
My header.php file is this:

<?php $header = "header.html"; $file = $_SERVER['SCRIPT_FILENAME']; readfile($header); readfile($file); ?>

And header.html is the html code for the header.

Any help would be greatly appreciated.