MYSQL all queries and connection using php oop approach

$servername = "localhost";
$username = "root";
$password = "";
$db="mydatabase";
$conn = new mysqli($servername, $username, $password,$db);// Create connection
if ($conn->connect_error) { // Check connection
    die("Connection failed: " . $conn->connect_error);
}
echo "Connected successfully";
################# queries  create,insert,update#################
$sql = "CREATE DATABASE myDB"; //create databsase

$firstname = $mysqli -> real_escape_string($_POST['firstname']);
$sql = "INSERT INTO MyGuests (firstname, lastname, email)
VALUES ($firstname , 'Doe', 'john@example.com')"; //insert data

$sql = "UPDATE MyGuests SET lastname='Doe' WHERE id=2"; //
if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}
################# queries for selecting data#################
$sql = "SELECT id, firstname, lastname FROM MyGuests";
$result = $conn->query($sql);
if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        echo "id: " . $row["id"]. " - Name: " . $row["firstname"]. " " . $row["lastname"]. "<br>";
    }
} else {
    echo "0 results";
}
$conn->close(); //release resources