Issues with exec()/system()

I'm having a bit of an issue using exec() and system(). Background: I have this script that installs forum software for a client, and that works like a champ. In it I use system() to copy the the default forum directory to the specified location the client sets, along with a unique directory name for the forum. The code looks like: $dir="unique_dir"; system("cp -R forum /path/of/forum/location/of/forums/".escapeshellarg($dir)); The forum directory is located in the same directory as the script. Now, this is where I run into trouble. When they delete directories, I want to backup the directory before I delete it just in case they delete the wrong information. I just can't get it to work. I've tried exec() and system() to no avail. Of course, I'm under the impression I can use tar via exec(). Currently I have: $dir="unique_dir"; $cmd="tar -cf /location/of/tar/files/$dir.tar /location/of/forum/diretory/*"; exec($cmd); This should work shouldn't? Any thoughts, suggestions? Anyone else done something like this? TIA Tom