//getting the number of rooms already booked on particular dates
$query = "SELECT count(*)
    FROM reservation
    WHERE (RoomID = '$ID' AND (Arrival <= '$check_out' AND Departure >= '$check_in'))";

$result = mysql_query($query);

if($result){
    $No_of_Rooms = mysql_fetch_assoc($result);
    $FreeRooms = $Total_Rooms - $No_of_Rooms; //-->error redirected here
    var_dump($No_of_Rooms);
    var_dump($FreeRooms);
}