I believe WestHost is using PHP 4.3.1. It could be different on your server. You can always check by making a file (e.g. test.php) with the following code in it, uploading it and then viewing it in your browser:
<? phpinfo(); ?>
If you are not familiar with SSH then installing a library (or installing anything for that matter) can be an extremely daunting task. If you aren't on WestHost 2.0, then you shouldn't even bother because it probably won't allow you. I'd recommend another way...
I've never tried doing what you want (not with user input anyway), but you can use the unzip (shell) program available on the server with your PHP script. Have a look at this code (works for me):
<?
exec('unzip images.zip', $outputlines);
echo 'Script returned the following: ';
foreach ($outputlines as $outputline) echo $outputline;
?>
The above unzips images.zip in the same directory and then sends the output of the command to the browser. Of course, that is oversimplified and you should never allow user input to control something like that, so you should take a look at this: http://www.php.net/manual/en/function.exec.php and incorporate some error checking, etc. in your script.