Singleton pattern in php - php

Question

class SingleTon
{
private static $instance;
private function __construct()
{
}
public function getInstance() {
if($instance === null) {
$instance = new SingleTon();
}
return $instance;
}
}
The above code depicts Singleton pattern from this article. http://www.hiteshagrawal.com/php/singleton-class-in-php-5
I did not understand one thing. I load this class in my project, but how would I ever create an object of Singleton initially. Will I call like this Singelton :: getInstance()
Can anyone show me an Singleton class where database connection is established?

Answer 1

An example of how you would implement a Singleton pattern for a database class can be seen below:
class Database implements Singleton {
private static $instance;
private $pdo;
private function __construct() {
$this->pdo = new PDO(
"mysql:host=localhost;dbname=database",
"user",
"password"
);
}
public static function getInstance() {
if(self::$instance === null) {
self::$instance = new Database();
}
return self::$instance->pdo;
}
}
You would make use of the class in the following manner:
$db = Database::getInstance();
// $db is now an instance of PDO
$db->prepare("SELECT ...");
// ...
$db = Database::getInstance();
// $db is the same instance as before
And for reference, the Singleton interface would look like:
interface Singleton {
public static function getInstance();
}

Answer 2

Yes, you have to call using
SingleTon::getInstance();
The first time it will test the private var $instance which is null and so the script will run $instance = new SingleTon();.
For a database class it's the same thing. This is an extract of a class which I use in Zend Framework:
class Application_Model_Database
{
/**
*
* #var Zend_Db_Adapter_Abstract
*/
private static $Db = NULL;
/**
*
* #return Zend_Db_Adapter_Abstract
*/
public static function getDb()
{
if (self::$Db === NULL)
self::$Db = Zend_Db_Table::getDefaultAdapter();
return self::$Db;
}
}
Note: The pattern is Singleton, not SingleTon.

Answer 3

A few corrections to your code. You need to ensure that the getInstance method is 'static', meaning it's a class method not an instance method. You also need to reference the attribute through the 'self' keyword.
Though it's typically not done, you should also override the "__clone()" method, which short circuits cloning of instance.
<?
class Singleton
{
private static $_instance;
private function __construct() { }
private final function __clone() { }
public static function getInstance() {
if(self::$_instance === null) {
self::$_instance = new Singleton();
}
return self::$_instance;
}
}
?>
$mySingleton = Singleton::getInstance();
One thing to not is that if you plan on doing unit testing, using the singleton pattern will cause you some difficulties. See http://sebastian-bergmann.de/archives/882-Testing-Code-That-Uses-Singletons.html

Answer 4

class Database{
private static $link=NULL;
private static $getInitial=NULL;
public static function getInitial() {
if (self::$getInitial == null)
self::$getInitial = new Database();
return self::$getInitial;
}
public function __construct($server = 'localhost', $username = 'root', $password ='tabsquare123', $database = 'cloud_storage') {
self::$link = mysql_connect($server, $username, $password);
mysql_select_db($database,self::$link);
mysql_query("SET CHARACTER SET utf8", self::$link);
mysql_query("SET NAMES 'utf8'", self::$link);
return self::$link;
}
function __destruct(){
mysql_close(self::$link);
}
}