How can I run a query at certain times?

Question

I have a query I need to pitch even for once . With this method , but this method did use a lot of CPU and slows the server .

 public function setRunningLevelsReadyToRun(){
    $interval=5; //seconds
    set_time_limit(0);        
    while(true) {
        $now=time();
        mysql_query("UPDATE `levels` SET status='running' WHERE 'NULL' not in (uid1,uid2,uid3,uid4) AND status = 'opened'") or die(mysql_error());
        sleep($interval-(time()-$now));
    }

}

I read this question but I want to run this method in separate thread.