add a simple example with a null logger

NullObject/PrintLogger.php

@@ -0,0 +1,20 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * PrintLogger is a logger that prints the log entry to standard output
+ */
+class PrintLogger implements LoggerInterface
+{
+
+    public function log($str)
+    {
+        echo $str;
+    }
+
+}