Thanks for the replies.
I was working on a (client) module that lists all .zip and .tgz files in the users folders. The user selects the file they want to decompress (using radio buttons) and the script decompresses the file (I had read that individuals were having problems with the permissions of decompressed files). However when the form was submitted, and I tested the $_POST variable, it was coming up with an empty array.
I guess I'd been doing something wrong with the form variables.
I got around the problem by switching to $_GET as I only wanted to pass a single filepath back to the script. It's a bit 'cluncky', but it does the job . . .
. . . except because I'm using the shell to extract the files, the permissions are all messed up
Thank goodness for the 'Fix Permission' button in the admin Panel.
Thanks for taking the time to reply, though - at least I know that I can use $_POSTed data.