String data conversion: Inserting data from Spinner using mysql - php

Question

I am trying to add data into MySQL database through spinner that has several options to choose. I do not know how to state value from spinner into MySQL code. The error occurs on the 39th line, which first line of Inserting Into database using MySQL code. I look forward to hearing from you soon.
<?php
include 'db.php';
$user_type = ['user_type'];
$first = $_POST['first'];
$last = $_POST['last'];
$email = $_POST['email'];
$username = $_POST['username'];
$password = $_POST['password'];
$phone = $_POST['phone'];
if(/*$user_type == '' ||*/ $first == '' || $last == '' || $email == '' || $username == '' || $password == '' || $phone == '')
{
echo 'Please, fill all fields';
} else{
$sql = "SELECT * FROM users WHERE user_username='$username'";
$check = mysqli_fetch_array(mysqli_query($conn,$sql));
if(isset($check))
{
echo 'Username already exist. Please, use another one.';
}else{
$hashedPWD = password_hash($password , PASSWORD_DEFAULT); //hash password
$sql = "INSERT INTO users (user_type, user_first, user_last, user_email, user_username, user_pwd, user_phonenumber) VALUES('$user_type','$first','$last','$email','$username','$hashedPWD','$phone')";
if(mysqli_query($conn,$sql))
{
echo 'Successfully registered';
} else{
echo 'Oops! Please try again!';
}
}
mysqli_close($conn);
}