MySQL Max() Function

I wonder if I'm missing something or doing something stupid. I'm trying to get the highest value of a field (it's in an "external" table and not part of the $values array) Running the sql directly brings up the correct result. No errors come up, it just doesn't provide the data requested. Any help would be gratefully received:-
function BeforeAdd(&$values,&$message,$inline)

{
$str = "SELECT users.email FROM users WHERE users.username = '".$_SESSION["OwnerID"]."'";

$rs = db_query($str,$conn);

$data = db_fetch_array($rs);

$email = $data["email"];
//The part above works perfectly
//The part below doesn't

$str1 = "SELECT Max(booking.BookingID) AS BookingID FROM booking";

$rs1 = db_query($str1,$conn);

$data1 = db_fetch_array($rs1);

$BookID = $data["BookingID"];

//everything below here works too - except the $BookID variable
$message="Your Classical Movement Quotation ID '$values[QuoteID]' has been booked. Your booking reference is:-". $BookID. "You can view the booking online.";

$subject="Thank you. Your Classical Movement booking is confirmed.";

$headers="From:booking@classicalmovement.co.uk"."\n\r";
foreach($values as $field=>$value)
$message.= "\n". $field." : ".$value. "\n";
mail($email, $subject, $message, $headers);

}