Remove vars from memory?

Hi all.
Will PHP remove variables from memory when script will NOT use them in rest
of code?
for example:

<?php
$tmp=true;
if ($tmp)  // here is $tmp used last time, will it be removed from memory?
...
... // $tmp is NOT in the rest of code
...
...
?>

or I must use allways unset()?

Thanx

Emo