you may wan't to unset all variables which are defined, here's one way:

<?php

function unset_all_vars($a)
{ foreach($a as $key => $val)
  { unset($GLOBALS[$key]); }
  return serialize($a); }

unset_all_vars(get_defined_vars());

?>

you can also save than a serialized var of the "memory" and perhaps store this in a temporary file.. very usefull if you work with text files and/or file uploads when you've got very large variables.

greetz