Classes definitions can now include static members (properties), accessible via the class. Common usage of static members is in the Singleton pattern.

PHP Code:

class Singleton {
    static private $instance = NULL;

    private function __construct() {
    }

    static public function getInstance() {
        if (self::$instance == NULL) {
            self::$instance = new Singleton();
        }
        return self::$instance;
    }
} 


The Singleton is one of the simplest Patterns to understand. It's common usage is to ensure that only one instance of a class is ever instantiated. The reason for wanting such behaviour varies but typically it is because only one object instantiated from the source class is required and you want the resulting object to be available throughout an application, i.e. globally available.

An example might be a class for storing Settings. A Settings class is a good candidate for a Singleton because its data is immutable (the only way to change settings data is to edit the settings file) and it is likely required in many areas of an application. Furthermore creating a new Settings object wherever it is needed is wasteful of resources - each is identical to all others.