PHP singleton inheritance

Prior to PHP 5.3, here is how we implement singleton:

class Service
{
private static $_instance = null;
private function __construct(){};
private function __clone(){};

public static function getInstance()
{
if (self::$_instance === null) {
self::$_instance = new self;
}
return self::$_instance;
}
}

However, we all know that it is very tricky if we want to extend/inherit this singleton class.
PHP 5.3 introduces LSB, which stands for Late Static Binding. It solves our problem of inheriting a singleton class. In PHP 5.3, we implement singleton in this way

class Service
{
private static $_instance = null;
private function __construct(){};
private function __clone(){};

public static function getInstance()
{
if (static::$_instance === null) {
static::$_instance = new static;
}
return static::$_instance;
}
}

Simply replace the 'self' keyword with 'static', we can inherit a singleton just like usual inheritance.

The difference is self keyword is bound to the referenced property or method at compile time. The self keyword points to the containing class and is unaware of subclasses. The static keyword forces PHP to bind to the code implementation at the latest possible moment.