Hi M Kovacs
I too faced the same problem on Solaris 10.
The problem as I found out was that exec() method was not working properly.
This was because the configuration file for PHP called php.ini has a security setting called "safe_mode"
If this is set to "on" then exec() will not execute the unzip program.
set it to off as in
safe_mode = off
restart your server and it should work now
check the following site about the php.ini settings:
Hope this helps