Rename Uploaded File

Hoi,
ik ben bezig met een avatar upload in te bouwen in mijn registreer script maar zou nu een random naam voor die afbeelding willen.
De random naam is geen probleem maar hoe moet je in hemelsnaam een afbeelding hernoemen?
Heb niet echt ervaring met files uploaden enzo..
Kan iemand me helpen?

(dit script is van jan koehoorn, hoop dat ik dit mocht gebruiken)

<?php

// © Jan Koehoorn Upload Script
// START

$config = array (
'upload_allowed_extensions' => array ('jpg', 'gif', 'png'),
'upload_allowed_mimetypes' => array ('image/jpeg', 'image/pjpeg', 'image/gif', 'image/png', 'image/x-png'),
'upload_max_filesize' => (1024 * 200),
'server_upload_dir' => $_SERVER['DOCUMENT_ROOT'] . '/!PROJECTS/beta/upload/'
);

$errs = array ();
$msgs = array ();
$files = array ();

if ($_SERVER['REQUEST_METHOD'] == 'POST') {
if (!$_FILES['avatar']['error']) {
$pathinfo = pathinfo ($_FILES['avatar']['name']);
if (!in_array ($pathinfo['extension'], $config['upload_allowed_extensions'])) {
array_push ($errs, 'Extension <strong>' . $pathinfo['extension'] . '</strong> not allowed');
}
if (!in_array ($_FILES['avatar']['type'], $config['upload_allowed_mimetypes'])) {
array_push ($errs, 'MIME type <strong>' . $_FILES['avatar']['type'] . '</strong> not allowed');
}
if ($_FILES['avatar']['size'] > $config['upload_max_filesize']) {
array_push ($errs, 'File too big. Maximum size: ' . ($config['upload_max_filesize'] / 1024) . ' KB');
}
if (!is_uploaded_file ($_FILES['avatar']['tmp_name'])) {
array_push ($errs, 'File is not an uploaded file');
}
if (empty ($errs)) {
if (!move_uploaded_file ($_FILES['avatar']['tmp_name'], $config['server_upload_dir'] . $_FILES['avatar']['name'])) {
array_push ($errs, 'Unable to move the uploaded file to it\'s destination folder');
}
else {
chmod ($config['server_upload_dir'] . $_FILES['avatar']['name'], 0644);
array_push ($msgs, 'Uploaded: <strong>' . $_FILES['avatar']['name'] . '</strong> (' . round (($_FILES['avatar']['size'] / 1024), 1) . ' KB)');
}
}
}
}
/*
foreach (glob ($config['server_upload_dir'] . '*.*') as $file) {
array_push ($files, basename ($file));
sort ($files);
}
*/
// EINDE

?>