When trying (no 'pear') to enumerate mounted drives on a win32 platform (Win XP SP3, Apache/2.2.11, PHP/5.2.9), I used:
<?php
function echo_win_drives() {
for($c='A'; $c<='Z'; $c++)
if(is_dir($c . ':'))
echo $c . ': ';
}
?>
which yielded:
A: C: D: E: F: G: H: I: