PHP - How do I replace this Singleton? - Custom DBAL - php

Question

I am rather new to the concepts of design patterns and I'm thinking of using Dependency Injection and Polymorphism (as each are applicable) -- but I have many Singletons and while most of them can easily be changed, my DBAL cannot.
The reason why is the DBAL creates a connection to the Database -- sets up it's own PDO object. If I passed new DBALs to every class that needs it (quite a few) I would get multiple, unnecessary connections to the database.
The class is something like this
class DB {
/**
* Hold the PDO object
* #var PDO
*/
private $_db;
/**
* Hold the last error messages
* #var string
*/
private $_error_message = NULL;
/**
* Hold the last error code
* #var int
*/
private $_error_code = NULL;
/**
* Connects to the database server and selects a database
*
* #param string $user MySQL database user
* #param string $password MySQL database password
* #param string $name MySQL database name
* #param string $host MySQL database host
* #return bool
*/
public function connect( $user, $password, $name, $host ) {
// Connect
try {
$this->_db = new PDO( "mysql:host=$host;dbname=$name", $user, $password );
} catch ( PDOException $e ) {
$this->_error_message = $e->getMessage();
$this->_error_code = $e->getCode();
return false;
}
return true;
}
// ...
}
?>
There will be many classes that inherit this class -- what is the best way to handle this? (I am new to design patterns)

Answer 1

An alternative method is to use a registry:
$db = new DB($host, $user, $pass);
Config::set('db', $db);
// Inside other classes
Config::get($this, 'db');
// Passes $this so the config can override the DB for different classes
Problem here is you end up with a Config singleton.
To truly do DI, you basicly need to pass object around to every other object.
$db = new DB($host, $user, $pass);
$user = new User($db);
// Or with a DI container
$c = new Pimple();
$c['db'] = function() {
return new DB($host, $user, $pass);
};
But ask yourself why you don't want to use a singleton.
If it looks like a singleton, smells like a singleton, and you use it like a singleton, then a singleton pattern is probably the best fit for the job.
http://pimple.sensiolabs.org/

Answer 2

add to the class:
private function __construct($user, $password, $name, $host ){
connect( $user, $password, $name, $host );
}
public static function getInstance(){
if(self::$_db == NULL) self::$_db = new DB;
return self::$_db;
}
and change the following:
// change $_db to be static!
private static $_db = NULL;