Check user name and password against database in PHP

<?php/*from www. j  a  v a 2s.c o  m*/

  session_start();

  // Has a session been initiated previously?
  if (! isset($_SESSION['username'])) {

      // If no previous session, has the user submitted the form?
      if (isset($_POST['username']))
      {

        $db = new mysqli("localhost", "webuser", "secret", "corporate");

        $stmt = $db->prepare("SELECT first_name FROM users WHERE username = ? and password = ?");

        $stmt->bind_param('ss', $_POST['username'], $_POST['password']);

        $stmt->execute();

        $stmt->store_result();

        if ($stmt->num_rows == 1)
        {

          $stmt->bind_result($firstName);

          $stmt->fetch();

          $_SESSION['first_name'] = $firstName;

          header("Location: http://www.example.com/");

        }

      } else {
        require_once('login.html');
      }

  } else {
    echo "You are already logged into the site.";
  }

?>

/*
CREATE TABLE users (
   id INTEGER UNSIGNED NOT NULL AUTO_INCREMENT,
   first_name VARCHAR(255) NOT NULL,
   username VARCHAR(255) NOT NULL,
   password VARCHAR(32) NOT NULL,
   PRIMARY KEY(id)
);

*/