Enable HSTS for php.net

[TimWolla]

The php.net properties don't have HTTP Strict-Transport-Security enabled. Since we are not going to suddenly disable TLS entirely, we should the header to prevent fallbacks to unencrypted HTTP once a user successfully visited php.net once.

[derickr]

As a first implementation, we should copy the line that we have for wiki.php.net:

        # HSTS (mod_headers is required) (15768000 seconds = 6 months)
        Header always set Strict-Transport-Security "max-age=15768000"