How to change permissions (CHMOD) of a file or a directory?

Answer: If you upload with File Manager all files/folders are uploaded/created by default with 755 permissions.
You can change the permissions of your files/folders only via FTP client. Most well-known FTP clients with a right click on a file/folder you can find an option for changing file/folder’s permissions.

Note: All php files need 755 permissions to work properly.