I think you missed the important notice in the documentation:
Rar Compression not supported
Due to restrictions with the Rar compression format, there is no compression available for free. When you want to compress files into a new Rar archive, you must provide a callback to the adapter that can invoke a Rar compression program.
This means, you can not create RAR files with plain PHP. (ZF is also plain PHP)
You need an external (command line) tool / program the create a RAR file with compression.
See also: zip - add files to rar archive using php - Stack Overflow