[Admin]

You give the fields aliases (a MySQL term), like so:

$test = $DB_site->query("
    SELECT [high]forum.title AS forumtitle[/high],user.userid,[high]thread.title AS threadtitle[/high],threadid,thread.lastpost,lastposter,replycount,views,postuserid,postusername 
    FROM thread 
    LEFT JOIN forum USING (forumid)
    LEFT JOIN user ON thread.lastposter=user.username
    ORDER BY lastpost DESC LIMIT 5
");

And then use $row[forumtitle] and $row[threadtitle].