Symbiose-unar is a command line tool provided to unarchive .ar files. It can be used in the terminal and in your own apps.
Pllace unar.php in the /usr/bin folder and download phpAr from GitHub (use at least the version 1.1), rename it phpAr.php, and put it in the /usr/lib/unar folder.
unar [FILE]
Symbiose-unar is released under GNU GPL v3 licence