Batch Rename PHP

#!/usr/bin/php
<?php

$files = glob("./backup-*.tar.gz");
foreach ($files as $file) {
    $data = explode(".", $file);
    $month = substr($file, 9, 1); // Accounts for the ./backup-
    exec("mv -f {$file} _temp_{$data[2]}{$data[1]}{$month}.tar.gz");
}