Hi,
depending on the method you transfer the form data to your php script (POST or GET) you have an array called $HTTP_POST_VARS or $HTTP_GET_VARS. If you loop through this array you can get all the variables from the form, in your special case I would try something like:
for($i=1;$i<101;$i++)
{
if(isset($HTTP_POST_VARS[$i]))
{
echo "checkbox variable name is: ".$i;
}
}
hope that helps you a bit !
cya Rob