Hi Enzo.
It seems like your PHP include_path is different when run from the command line versus running in a web server.
In my server, Archive_Tar is in /usr/local/share/pear/Archive/Tar.php, and for PHP to find it I had to add this line to /etc/php.ini
include_path = ".:/usr/local/share/pear"
Your configuration could be quite different. I’m sorry I can’t offer much more assistance than this. You might be able to get more information by using the phpinfo() function, as @asmecher described here.