Question

In PHP, I have following Singleton Database Class:

class Database
{
    private static $instance;

    private function __construct()
    {
        self::$instance = new mysqli('localhost', 'root', 'Matthias', 'financecontrol', '3307');
        if (!self::$instance) {
            throw new Exception('Could not connect to database in function __construct.');
        }
    }

    public static function getInstance()
    {
        if (!self::$instance) {
            self::$instance = new Database();
        }
        return self::$instance;
    }

}

Whenever I try to perform a query on the database in another PHP file, for example to check whether a user already exists:

function userExists($username)
    {
        try {
            $connection = Database::getInstance();

            $result = $connection->query("select * from user where username='$username'");
            if (!$result) {
                throw new Exception("Connection to database failed in function userExists.");
            }
            if ($result->num_rows > 0) {
                return true;
            } else {
                return false;
            }
        } catch (Exception $ex) {
            $errorPager = new ErrorpageGenerator();
            $errorPager->generateErrorPage($ex->getMessage());
            return false;
        }
    }

I get an error message “PHP Fatal error: Call to undefined method Database::query() in User.php on line 44”

I’ve tried adding a query function in the Database class, but that did not seem to fix the problem. Any ideas? Thanks

Answer 1

You have to add this method of course. But you cannot assign Database() and the mySQLi object to m_pInstance

so do:

class Database
{
  private static $conn;

  // ...
  public function __construct() 
  {
     self::$conn = new mysqli('localhost', 'root', 'root', 'database', '3307');
     //...

and then

public function query($sql)
{
  return self::$conn->query($sql);
  // or 
  return mysqli_query(self::$conn, $sql);
}

EDIT

Working code:

class Database
{
  private static $instance = null;

  private static $conn;

  private function __construct()
  {
    self::$conn = new mysqli('localhost', 'root', 'root', 'database', '3307');
  }

  public static function getInstance()
  {
    if (self::$instance == null) {
      self::$instance = new Database();
    }

    return self::$instance;
  }

  public function query($sql)
  {
    return self::$conn->query($sql);
  }
}