Help meee

$sql = "SELECT * FROM users WHERE un=?;";
    $stmt = mysqli_stmt_init($conn);

    if(!mysqli_stmt_prepare($stmt, $sql)) {
        header('Location: register.php?sql=db_connection_error');
        exit();
    }

    else {

        mysqli_stmt_bind_param($stmt, 's', $username);
        mysqli_stmt_execute($stmt);

        if($username === $sql) {
            header('Location: register.php?username=taken');
            exit();
        }

    }