For instance on the above I would do:

Db::init($host,$user,$password,$database);
Db::getInstance()->connect();
Db::getInstance()->selectDb();
$sql = ‘SELECT * FROM users’;
Db::getInstance()->query($sql);

This splits the implementation in different methods and you can do the lazy connect with a lot of flexibility. Instead of connecting to the server and selecting the database you can now reuse the connection to the same database server but different database. All you have to do is call

Db::getInstance()->selectDb($database);

The additional code that I am thinking is below:

private static $_host = ”;
private static $_user = ”;
private static $_pass = ”;
private static $_database = ”;

public static function init($host, $user, $password, $database)
{
$self::_host = $host;
$self::_user = $user;
$self::_pass = $password;
$self::_database = $database;
}

public function connect()
{
$this->_conn = mysql_connect($this->_host, $this->_user, $this->_pass);
if (!$this->_conn) {
throw new Exception(‘Cannot connect to the database :’ . mysql_error());
}
}

public function selectDb($database = ”)
{
if (!$this->_conn) {
throw new Exception(‘No database connection. Please connect to a server first.’);
}

$this->_database = ($database) ? $database : $this->_database;

$db = mysql_select_db($this->_database, $this->_conn);
if (!$db) {
throw new Exception(‘Cannot select the database : ‘ . mysql_error());
}
}

Kinda like this.. (lets hope this shows up ok)

Db::connect(‘user’, ‘pass’, ‘db’, ‘host’);

class Db {
  public static connect($user, $pass, $db, $host) {
    $this->_user = $user;
    //etc.
  }

  public static getInstance() {
    //if no connection, use vars set in connect to open new
  }
}

You would then initialize the database connection using the connect method somewhere in your code. Assuming you’re including a file or routing all calls through index.php, it could be run there and the rest of the code does not need to care about it.