Sending options from html select box into database

Question

Update: I’m now getting an error telling me that roomID is null and this is why it’s not going into the database. Can you see why it would be null when it should be the option that the user selects in the select box?

I'm working on a hotel booking system as an exercise. I am trying to make a form that will insert a new booking into the database. I have the customer and room IDS in the mySQL database and I have put them into HTML select boxes for the user to select an option.

This works fine until I try to save the new booking to the database. I don't get any errors and it tells me the booking has been saved on my page but if I go into the database it hasn't been saved. I have tried inserting the customerID and roomID values from text boxes so just typing it in instead of the drop down menu and that inserts into the database just fine.

I'm pretty sure the problem is something to do with getting the value out of the select box and into the POST variable but I'm not 100% sure. I've only been coding for 6 months so I don't really know what I'm doing! I've tried to figure it out by looking at other examples on here but nothing seems to work.

Here's my code:

Getting the data to fill the select boxes:

//Locate room names
$query = $query = 'SELECT `roomID` FROM `room`';
$result2 = mysqli_query($DBC,$query);
$rowcount = mysqli_num_rows($result2);
if ($rowcount > 0) {
  $row = mysqli_fetch_assoc($result2);}

//Locate customerIDs
$query = $query = 'SELECT `customerID` FROM `customer`';
$result = mysqli_query($DBC,$query);
$rowcount = mysqli_num_rows($result);
if ($rowcount > 0) {
  $row = mysqli_fetch_assoc($result);}

Creating Select boxes:

<form method="POST" action="newbooking.php">
<p>
  <label for="customerID">Customer ID: </label>
<?php
  echo "<select name='customerID'id='customerID' input type='number'>";
        // output data of each row
        while($row = $result->fetch_assoc()) {
          echo "<option value='customerIDselect'>" . $row['customerID'] . "</option>";
          //customerID
       $customerID = cleanInput($_POST['customerIDselect']); 
        }
        echo "</select>";
 ?>
  </p> 
 <p>
  <label for="rooms">Room (name, type, beds): </label>
<?php
  echo "<select name='roomID'id='roomID' input type='number'>";
        // output data of each row
        while($row = $result2->fetch_assoc()) {
          echo "<option value='roomIDselect'>" . $row['roomID'] . "</option>";
          //roomID
       $roomID = cleanInput($_POST['roomIDselect']);
        }
        echo "</select>";
 ?>
  </p>  

Inserting data into database:

//save the customer data if the error flag is still clear
    if ($error == 0) {
        $query = 'INSERT INTO `bookings` (`customerID`,`roomID`,`checkin`,`checkout`, `extras`) VALUES (?,?,?,?,?)';
        $stmt = mysqli_prepare($DBC,$query); //prepare the query
        mysqli_stmt_bind_param($stmt,'sssss', $customerID, $roomID, $checkin, $checkout, $extras); 
        mysqli_stmt_execute($stmt);
        mysqli_stmt_close($stmt);    
        echo "<h2>booking saved</h2>";        
    } else { 
      echo "<h2>$msg</h2>".PHP_EOL;
    }