php - Uncaught Error: Call to a member function query() on null

I got stuck for hours in the code below. I don't know how I can fix this error.

Notice: Undefined variable: mysqli in D:\xampp\htdocs\recon\register.php on line 19

Fatal error: Uncaught Error: Call to a member function query() on null in D:\xampp\htdocs\recon\register.php:19 Stack trace: #0 {main} thrown in D:\xampp\htdocs\recon\register.php on line 19

$conn = new mysqli('localhost', 'root', '', 'user');

if (mysqli_connect_errno()) {
    printf("Connect failed: %s\n", mysqli_connect_error());
    exit();
}


$uname = $_POST['uname'];
$psw = $_POST['psw'];
$options = [
'cost' => 12,];
$hashedpassword= password_hash($psw, PASSWORD_BCRYPT, $options);

$result = $mysqli->query("SELECT username FROM registration WHERE username = '$uname'"); 
$row_count = $result->num_rows; 
if($row_count == 1) { 

    echo 'User already exists, try another one.'; } 
else {

    $query = "INSERT INTO user (username, password) VALUES(?, ?)";
    $statement = $mysqli->prepare($query);

    $statement->bind_param('ss', $uname, $hashedpassword);

    if($statement->execute())
    {

         print 'Success! Last inserted record : ' .$statement->insert_id .'
'; 
    }
    else
    {
         die('Error : ('. $mysqli->errno .') '. $mysqli->error);
    }
    $statement->close();

}


?>

Answer

You are declaring instance of mysqli called $conn. This represents your connection to DB. You should call methods on variable $conn and not on (undefined) variable $mysqli. So ie. your line 19 should be:

$result = $conn->query("SELECT username FROM registration WHERE username = '$uname'")

Also to prevent SQL-Injection on your queries/web-pages you should use prepared statements EVERYWHERE(including SELECT).