<?php
function gen_uuid($len=8) {
    // Seed + uniqueness
    $hex = md5("secret" . uniqid("", true));
    $pack = pack('H*', $hex);
    $uid = base64_encode($pack);        // max 22 chars
    $uid = preg_replace("#[^A-Z1-9]#", "", strtoupper($uid));    // uppercase only

    while (strlen($uid)<$len)
        $uid = $uid . gen_uuid(22);     // append until length achieved

    return substr($uid, 0, $len);
}