DATABASE MYSQL CONNECTION AND QUERIES with PHP using PDO

$servername = "localhost";
$username = "root";
$password = "";
$db="mydatabase";

try {
    $conn = new PDO("mysql:host=$servername;dbname=$db", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected successfully"; 
}
catch(PDOException $e) {
    echo "Connection failed: " . $e->getMessage();
}

################# queries  create,insert,update#################

// create database
$sql = "CREATE DATABASE myDB";
$conn->exec($sql);

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

// update data
$sql = "UPDATE MyGuests SET lastname='Doe' WHERE id=2";
$conn->exec($sql);

################# queries for selecting data#################
$sql = "SELECT * FROM transections WHERE merchent_order_id = :m_id LIMIT 1";
$stmt = $conn->prepare($sql);
$stmt->bindParam(':m_id', '1');
$stmt->execute();
$results = $stmt->fetchAll(PDO::FETCH_ASSOC);
//OR
$results = $stmt->fetch(PDO::FETCH_ASSOC);
$conn = null;  // close the connection

################# Delete #################
$query = "DELETE FROM $table WHERE id = :id";
$statement = $pdo->prepare($query);
$statement->bindParam(':id', $id, PDO::PARAM_INT);