[php] upload immagini con nome univoco

[Base]

ok questo script mi funziona correttamente (fonte php.net)

$uploaddir = './foto/';
$uploadfile = $uploaddir . basename($_FILES['userfile']['name']);

echo '<pre>';
if (move_uploaded_file($_FILES['userfile']['tmp_name'], $uploadfile)) {
   echo "File is valid, and was successfully uploaded.\n";
} else {
   echo "Possible file upload attack!\n";
}

echo 'Here is some more debugging info:';
print_r($_FILES);

print "</pre>"; 

mi aiutate da qui?
come faccio a rinominare il file applicando md5(uniqid(microtime(), true));

help!