PHP mysqli prepare() Function

<?php
$mysqli = new mysqli("localhost","my_user","my_password","my_db");

// Check connection
if ($mysqli -> connect_errno) {
  echo "Failed to connect to MySQL: " . $mysqli -> connect_error;
  exit();
}

// prepare and bind
$stmt = $mysqli -> prepare("INSERT INTO MyGuests (firstname, lastname, email) VALUES (?, ?, ?)");
$stmt -> bind_param("sss", $firstname, $lastname, $email);

// set parameters and execute
$firstname = "John";
$lastname = "Doe";
$email = "john@example.com";
$stmt -> execute();

$firstname = "Mary";
$lastname = "Moe";
$email = "mary@example.com";
$stmt -> execute();

echo "New records created successfully";

$stmt -> close();
$mysqli -> close();
?>