The setting of $rooturl = "http://$_SERVER['HTTP_HOST'] only allows you to set https or http . No casewise SSLencryption.

Often installations are using selfsigned certs which will produce cryptic messages beside adding load when just delivering surveys to respondents.

A workaround is leaving the $rooturl empty. But that is causing problems with e.g. links in emails incorrect.

To use rewrite url routine of the webserver is another.