Anda perlu menggunakan notasi titik:
foreach($horses as &$horse)
{
$newdata=array('$set'=>array('horses.odds'=> array($horses['odds'] => $price)));
$filter=array("marketId"=> $marketId);
$c->update($filter,$newdata);
}
Lihat http://www.mongodb.org/ tampilan/DOCS/Titik+Notasi+%28Mencapai+ke+Objek%29 .