HSTS

HTTP Strict Transport Security lets a web site inform the browser that it should never load the site using HTTP and should automatically convert all attempts to access the site using HTTP to HTTPS requests instead.

Links:

• wikipedia
• mozilla

PHP / Apache

<IfModule mod_headers.c>
  Header always set Strict-Transport-Security "max-age=63072000; includeSubDomains"
</IfModule>