In situations where you have multiple possible database connections, you could still use the singleton solution but it would absolutely be different in situations like the one that you’ve presented.

In situations where you have the possibility of multiple database connections, a singleton solution is still possible.

Realize, there are MULTIPLE other possibilities. The example I’m going to give (I do not believe it is the best) is only an example – it is not the only way…

The example strategy would be to have an array private to MyDBH that tracks all instances, and update MyDBH’s getInstnace method to accept the name of the db connection you want to create. In order to do this, either you’d have to define every database conection’s credentials in the MyDBH class, create a separate class that the MyDBH extends for storing DB configuration, create a configuration file that can be included to obtain an instances’s credentials, or create an object that stores this sort of information.

This example just creates a dbconnection factory because I’m trying to keep this short and sweet (and also show the factory pattern at the same time):
< ?php
// using the "i" prefix to indicate its an interface...
interface iDbConnection {
public function getDbInfo();
// anything else...
}

// Could make this abstract too obviously....
class DbConnectionFactory {
// just an example for factory as well...
public static function getInstance($db_connection) {
if(class_exists($db_connection) && method_exists()) {
$dbConnection = new $db_connection;
if(!($dbConnection instanceof iDbConnection)) {
throw new Exception("Invalid DB Connection Requested!");
}
// do whatever
return $dbConnection->getDbInfo();
}
}
}

class AdvertisementDbConnection implements iDbConnection {
private $db_info = array(“host” => “localhost”, “dbname” => “myAdvertisementDB”, “username” => “myAdvertisementDB_user”, “password” => “myAdvertisementDB_password”);

// whatever else

public function getDbInfo() {
return $this->db_info;
}
}

class MyDBH {
// changed this to an array…
private static $instances = array();

// …….
//
// same logic/methods/properties from above…
//
// …….

public static function getInstance($db_connection = ‘default’) {
// keep it all consistent…
$db_connection = strtolower($db_connection);
if(!isset(self::$instances[$db_connection])) {
$db_info = (strtolower($db_connection) == ‘default’ ? null : DbConnectionFactory::getInstance($db_connection));
self::$instances[$db_connection] = new MyDBH($db_info);
}
return self::$instances[$db_connection];
}
}

// same ole, same ole..
$dbh = MyDBH::getInstance();

// get advertisement
$dbh = MyDBH::getInstance(‘AdvertisementDbConnection’);

While crude, this shows a basic example.. You could improve this in many, many different ways. Up to you how to do this.

Also, I’d like to point out that multiple user’s on a page does not affect the singleton since PHP creates a new thread per user that accesses your page. In other words, the logic that checks for “current state” to display a page, and will still use everything you would normally expect.

Note: I didn’t actually test the above so feel free to make improvements where needed!