Add Singleton pattern code example

pages/Design-Patterns.md

@@ -68,7 +68,48 @@ yourself a lot of trouble down the road by using factories.
 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.
 
-**TODO: NEED NEW SINGLETON CODE EXAMPLE**
+{% highlight php %}
+<?php
+
+final class NotificationLogger {
+    private static ?self $instance = null;
+
+    // The constructor is private to prevent creating instances from outside
+    private function __construct()
+    {}
+
+    // Prevent cloning the instance
+    private function __clone(): void {}
+
+    // Prevent unserialisation which would lead to create a new instance (legacy PHP mechanism, kept for backward compatibility)
+    public function __wakeup(): void {
+        throw new \LogicException("Cannot unserialize singleton");
+    }
+
+    // Prevent unserialisation (modern PHP mechanism)
+    public function __unserialize(array $data): void {
+        throw new \LogicException("Cannot unserialize singleton");
+    }
+
+    // Creates new instance if it does not exist, otherwise it returns the existing instance
+    public static function getInstance(): self {
+        return self::$instance ??= new self();
+    }
+
+    public function log(string $message): void {
+        echo "[SINGLETON LOG] {$message}";
+    }
+}
+
+// Usage
+$logger1 = NotificationLogger::getInstance();
+$logger2 = NotificationLogger::getInstance();
+
+// Prove they are the same instance
+var_dump($logger1 === $logger2); // bool(true)
+$logger1->log("Application started");
+
+{% endhighlight %}
 
 The code above implements the singleton pattern using a [*static* variable](https://www.php.net/language.variables.scope#language.variables.scope.static) and the static creation method `getInstance()`.
 Note the following: