Hvis du tjekker php-manualen her:http://php.net/manual /fr/mysqli.select-db.php
Du vil se funktionen mysqli_select_db tag to parametre i input.
Så du burde have
// Connect to server and select databse.
$conn = mysqli_connect($host, $username, $password) or die("cannot connect");
mysqli_select_db($conn, $db_name) or die("cannot select DB");