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