Indsæt et stykke debug-kode i dit script for at bevise, at navnet er korrekt:-
// Select all bookings
$sql = "SELECT * FROM booking";
$result = mysqli_query($con,$sql);
echo "<p>";
while ($row = mysqli_fetch_array($result, MYSQLI_ASSOC)) {
print_r( $row ); // debug code
// echo $row['custName'] . " - " . $row['roomNb'] . " - " . $row['date'];
echo $row['custName'];
}