function get_total_adults()
{
    $sql = "SELECT SUM(number_adults_attending) as number_of_adults FROM is_nfo_rsvp";
    $result = mysql_query($sql) or die(mysql_error());
    $array = mysql_fetch_assoc($result);

    return $array['number_of_adults'];
}

function get_total_adults() {
    $sql = 'SELECT SUM(number_adults_attending) FROM is_nfo_rsvp';
    $result = mysql_query($sql) or die(mysql_error());
    // I'd throw a catchable exception (see below) rather than die with a MySQl error

    return mysql_result($result, 0);
}

function get_total_adults() {
    $sql = 'SELECT SUM(number_adults_attending) FROM is_nfo_rsvp';
    $result = mysql_query($sql);
    if (!$result) {
        throw new Exception('Failed to get total number of adults attending');
    }

    return mysql_result($result, 0);
}

try {
    $total_adults = get_total_adults();
} catch(Exception $ex) {
    die('Whoops! An error occurred: ' . $ex->getMessage());
    // or even better, add to your template and dump the template at this point
}
// continue code