forcing HSTS on PHP website

I'm running a PHP website and added following to my .htaccess file:

Header always set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload"

It seems however that running a PHP site this doesn't work using the .htaccess file. You have apparently to force https through the index.php file in the root having the beginning of the index file look like this:

<?php

header('Strict-Transport-Security: max-age=31536000');

Eveytime I republish my site I manually adjust the index.php on the webserver to force browsers to use https. I assume this could also be arranged in the index.php file in RapidWeaver or in the projectsettings, but I wasn't able to find out how.

Is this possible and where can I insert that code to be permanently in the index.php file on the root?