wp 4.9 comes with a bug... you can not edit any php file anymore in the backend theme or plugin editor !

This message occurs

"Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP."

It does not matter if the website is http or https... you just cannot edit any php file anymore and save it. https://i.imgur.com/xipanRo.jpg