You should change the PHP version requirement to 5.

You should change the PHP version requirement to 5.3. get_called_class() function does not exists in PHP 5-5.2

It`s true. Thx

Also, your solution is not complete.
Think about overriding your Singleton class. If you have something like that:

class MyCustomeSingleton extends Singleton {
public function __construct() {
return parent::__construct();
}
}

what's stopping me to do:

$s1= new MyCustomeSingleton();
$s1->test();

$s2= new MyCustomeSingleton();
$s2->test();

To avoid this, you have the simple solution by marking constructor & clone magic method from Singleton class as final (or entire class Singleton if you don't want to be extended anymore).

To be a class a singleton, basically you should follow 3 rules:
1. constructor shouldn't be public
2. inhibit the clone operator
3. do not allow first two points to be changed by overriding.

Mihai