<form action="loginuser.php" method="POST">
<div class="form-group">
    <select name="userList">
        <option> - - - </option>

        <?php
        include 'db.php';

        $sql = "SELECT username FROM dbusers";
        $result = $conn->query($sql);

        if ($result->num_rows > 0) {

            while($row = $result->fetch_assoc()) {
                echo "<option>" . $row["username"]. "</option>";
            }
            }
        ?>
    </select>
</div>
<div class="form-group">
    <input name "password" type="password" class="form-control" id="inputPassword">
</div>
<div class="form-group">
    <button type="submit" class="btn btn-info btn-sm btn-block">Login</button>
</div>

<?php
session_start();
require('db.php');

if (isset($_POST['username']) and isset($_POST['password'])){
    echo "TEST - GOOD";

$username = $_POST['username'];
$password = $_POST['password'];

$query = "SELECT * FROM `dbusers` WHERE username='$username' and password='$password'";

$result = mysqli_query($conn, $query) or die(mysqli_error($conn));
$count = mysqli_num_rows($result);

if ($count == 1){
$_SESSION['username'] = $username;
}else{

$fmsg = "Invalid Login Credentials.";
}
}
if (isset($_SESSION['username'])){
$username = $_SESSION['username'];
echo "Succes";

}else{
    echo "TEST - FALSE";
}
?>