The unset( ) function deletes a variable so that isset( ) will return false. Once deleted, you can recreate a variable later on in a script.
$name = "Paul";
if (isset($name)) print "Name is set\n";
unset($name);
if (isset($name)) print "Name is still set\n";
?>
That would print out "Name is set", but not "Name is still set", because calling unset( ) has deleted the $name variable.