Just create an .htaccess file and add the HSTS header app-root/repo/php.
# Force https
RewriteEngine on
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule .* https://%{HTTP_HOST}%{REQUEST_URI} [R,L]
# Add HSTS header
Header set Strict-Transport-Security "max-age=31536000"
via Chu's Space http://bit.ly/1UfSpZ4