Enhanced the example for the Singleton design pattern.

- Moved static variable into the static creation method getInstance.
- Replaced "new self();" with "new static" (late static binding to allow
subclassing).
- Changed visibility of __construct to protected.
- Added the two private methods __clone and __wakeup.
- Added DocBlock comments.
- Updated description and enhanced with links to the PHP manual.

pages/Design-Patterns.md

@@ -71,30 +71,72 @@ only one instance of a particular class. The singleton pattern enables us to do
 <?php
 class Singleton
 {
-    static $instance;
+    /**
+     * Returns the *Singleton* instance of this class.
+     *
+     * @staticvar Singleton $instance The *Singleton* instances of this class.
+     *
+     * @return Singleton The *Singleton* instance.
+     */
+    public static function getInstance()
+    {
+        static $instance = null;
+        if (null === $instance) {
+            $instance = new static;
+        }
 
-    private function __construct()
+        return $instance;
+    }
+
+    /**
+     * Protected constructor to prevent creating a new instance of the
+     * *Singleton* via the `new` operator from outside of this class.
+     */
+    protected function __construct()
     {
     }
 
-    public static function getInstance()
+    /**
+     * Private clone method to prevent cloning of the instance of the
+     * *Singleton* instance.
+     *
+     * @return void
+     */
+    private function __clone()
     {
-        if (!isset(self::$instance)) {
-            self::$instance = new self();
-        }
+    }
 
-        return self::$instance;
+    /**
+     * Private unserialize method to prevent unserializing of the *Singleton*
+     * instance.
+     *
+     * @return void
+     */
+    private function __wakeup()
+    {
     }
 }
 
-$instance1 = Singleton::getInstance();
-$instance2 = Singleton::getInstance();
+class SingletonChild extends Singleton
+{
+}
 
-echo $instance1 === $instance2; // outputs 1
+$obj = Singleton::getInstance();
+\var_dump($obj === Singleton::getInstance());             // bool(true)
+
+$anotherObj = SingletonChild::getInstance();
+\var_dump($anotherObj === Singleton::getInstance());      // bool(false)
+
+\var_dump($anotherObj === SingletonChild::getInstance()); // bool(true)
 {% endhighlight %}
 
-The code above implements the singleton pattern using a statically scoped variable and the `getInstance()` method.
-Note that the constructor is declared as private to prevent instantiation outside of the class via `new` keyword.
+The code above implements the singleton pattern using a [*static* variable](http://php.net/language.variables.scope#language.variables.scope.static) and the static creation method `getInstance()`.
+Note the following:
+
+* The constructor [`__construct`](http://php.net/language.oop5.decon#object.construct) is declared as protected to prevent creating a new instance outside of the class via the `new` operator.
+* The magic method [`__clone`](http://php.net/language.oop5.cloning#object.clone) is declared as private to prevent cloning of an instance of the class via the [`clone`](http://php.net/language.oop5.cloning) operator.
+* The magic method [`__wakeup`](http://php.net/language.oop5.magic#object.wakeup) is declared as private to prevent unserializing of an instance of the class via the global function [`\unserialize()`](http://php.net/function.unserialize).
+* A new instance is created via [late static binding](http://php.net/language.oop5.late-static-bindings) in the static creation method `getInstance()` with the keyword `static`. This allows the subclassing of the class `Singleton` in the example.
 
 The singleton pattern is useful when we need to make sure we only have a single instance of a class for the entire
 request lifecycle in a web application. This typically occurs when we have global objects (such as a Configuration