how to update a table record using php to sqlsrv

   function addActiveGuest($ip, $time){
 $config = $this->getConfigs();
      if(!$config['TRACK_VISITORS']) return;
      $sql =  $this->connection->prepare("REPLACE INTO ".TBL_ACTIVE_GUESTS." VALUES ('$ip', '$time')");
 $sql->execute();
      $this->calcNumActiveGuests();
 $time = timestamp;
   }

is the equiv of what i need in mysql i cannot figure this out i have about 4 hours on this already.