MYSQL all queries and connection using php mysqli approach

$servername = "localhost";
$username = "username";
$password = "password";
$db="mydatabase";
$conn = mysqli_connect($servername, $username, $password);// Create connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}
echo "Connected successfully";
################# queries #################
$sql = "CREATE DATABASE myDB"; //create databsase
$sql = "INSERT INTO MyGuests (firstname, lastname, email)
VALUES ('John', 'Doe', 'john@example.com')"; //insert data
$sql = "UPDATE MyGuests SET lastname='Doe' WHERE id=2"; //
if (mysqli_query($conn, $sql)) {
    echo "successfully EXECUTED";
} else {
    echo "Error: " . mysqli_error($conn);
}
################# queries #################
$sql = "SELECT id, firstname, lastname FROM MyGuests";
$result = mysqli_query($conn, $sql);
if (mysqli_num_rows($result) > 0) {
    // output data of each row
    while($row = mysqli_fetch_assoc($result)) {
        echo "id: " . $row["id"]. " - Name: " . $row["firstname"]. " " . $row["lastname"]. "<br>";
    }
} else {
    echo "0 results";
}
mysqli_close($conn); //release resources