A previous question just to understand what I’m doing: How to change permissions for a folder and its subfolders/files in one step
So if I change the chmod to 755 for chmod 75 /opt/lampp/htdocs and I restart the system I want the files to be in chmod 755.
Once set, Linux filesystem permissions are unaffected by rebooting the system, unless you have some non-standard software running at boot time which is going back to change them to some default.