PHP Singleton 101
Tuesday, May 10, 2011 EDT (Updated on: Tuesday, August 9, 2011 EDT)
by: Eric Potvin
In software engineering, the singleton pattern is a design pattern used to implement only one instantiation of a class to one object. This pattern might not be used often but it might be very helpful in database classes for example. We do not need to connect to a database every time a script is called if we use a persistent connection.
The singleton concept is simple: A class that can be instantiated only once. To achieve this, we cannot declare the constructor as public, but as private. Now, since a private method cannot be call from outside of the class, we need to create a static public method that as access to the constructor.
The concept is not that hard to understand and neither to implement. Here's a sample code:
class SingleTon {
private static $instance = NULL;
private function __construct() {
// use my mysql_pconnect()
}
public function __destruct() {
// have some useful stuff here
}
public static function getInstance() {
if(!is_object(self::$instance)) {
self::$instance = new SingleTon();
}
return self::$instance;
}
}
As you can see, the constructor is declared private. Therefore, you cannot to something like that:
$SingleTon = new SingleTon();You will get something like:
Fatal error: Call to private SingleTon::__construct() from invalid context in /home/eric/test.php on line xxIn order to make it work, you simply needs to do it like that:
$SingleTon = SingleTon::getInstance();That's it!
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