谁能解释一下这个PHP

<?php
require_once("./includes/upb.initialize.php");
require_once("./includes/class/upload.class.php");
require_once("./includes/api/usermanagement/authentication.class.php");
$auth = new UPB_Authentication($tdb);
$uploadRec = $tdb->get("uploads", $_GET["upload_id"]);
if($uploadRec !== false)
{
if($auth->access("topic", 'r', $uploadRec[0]["forum_id"], $uploadRec[0]["topic_id"]) == true)
{
$upload = new upload(DB_DIR, 0,$_CONFIG['fileupload_location']);
$upload->getFile((int) $_GET["upload_id"]);
// Update the download count
$upload->edit("uploads", (int) $_GET["upload_id"], array("downloads" => ((int)$upload->file["downloads"] + 1)));
$upload->dumpFile();
}
else 
{
exitPage("You do not have permission to download this file", true);
}
}
else 
{
exitPage("Upload does not exist", true);
}
?>