@xvisionbelgium lines 20 and 21 in config.php contain the following,
// Can Be Set Manually Like "http://localhost/mysite/".
define("SITE_ADDR", $site_addr);
which should work fine in any environment. please confirm that the mixed content error you are getting is not with a resource you added yourself to phprad. Setting this
define("SITE_ADDR", $site_addr);
on line 20 to this
define("SITE_ADDR", "https://localhost/mysite/");
should fix it. Please replace https://localhost/mysite/ with your own site link, make sure to include the https:// to it. Note that if the issue you are having is with images, then it means you might have configured phprad to store the image full path in the database, instead of the relative path.