具有多个选择选项的选择列表中的PHP回显

I'm having a bit of struggle putting together a selection list in HTML where the options or values are being retrieved from a table in the database. I get the following result:

As you can see this is not my intention, I rather have all options in 1 Selection List, with multiple options clickable.

This is my HTML Code (Bootstrap Framework):

<div class="container-fluid well well-lg">

     <?php while ($itemsrow = $query->fetch()) : ?>

<div class="form-group">

      <label for="sel1">Select list:</label>

<select class="form-control" id="sel1">

         <option><?php echo $itemsrow['Beschrijving']; ?></option>

The PHP Logic:

$conn = Db::getInstance();
$query = $conn->prepare("SELECT * FROM items WHERE user_id = $userID");
$query->execute();

Thanks in advance!

Currently, you're creating new elements inside the loop so yes, you will get a new select per iteration.

You should only keep the <option>-elements inside the loop:

<div class="container-fluid well well-lg">
    <div class="form-group">
        <label for="sel1">Select list:</label>

            <select class="form-control" id="sel1">

            <?php while ($itemsrow = $query->fetch()) : ?>  

                <option><?php echo $itemsrow['Beschrijving']; ?></option>

            <?php endwhile; ?>

You should put your loop into <select> tag

<select class="form-control" id="sel1">
    <?php while ($itemsrow = $query->fetch()) : ?>
        <option><?php echo $itemsrow['Beschrijving']; ?></option>