Added singleton pattern to the Design-Patterns page

pages/Design-Patterns.md

@@ -62,6 +62,50 @@ yourself a lot of trouble down the road by using factories.
 
 * [Factory pattern on Wikipedia](https://en.wikipedia.org/wiki/Factory_pattern)
 
+## Singleton
+
+When designing web applications, it often makes sense conceptually and architecturally to allow access to one and 
+only one instance of a particular class. The singleton pattern enables us to do this.
+
+{% highlight php %}
+<?php 
+class Singleton
+{
+    static $instance;
+
+    private function __construct()
+    {
+    }
+
+    public static function getInstance()
+    {
+        if (!isset(self::$instance)) {
+            self::$instance = new self();
+        }
+
+        return self::$instance;
+    }
+}
+
+$instance1 = Singleton::getInstance();
+$instance2 = Singleton::getInstance();
+
+echo $instance1 === $instance2; // outputs 1
+{% 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 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 
+class) or a shared resource (such as an event queue).
+
+You should be wary when using the singleton pattern, as by its very nature it introduces global state into your 
+application, reducing testability. In most cases, dependency injection can (and should) be used in place of 
+singleton objects.
+
+* [Singleton pattern on Wikipedia](https://en.wikipedia.org/wiki/Singleton_pattern)
+
 ## Front Controller
 
 The front controller pattern is where you have a single entrance point for you web application (e.g. index.php) that