adding the missing classes to show the real purpose of the pattern

Command/HelloCommand.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace DesignPatterns\Command;
+
+/**
+ * This concrete command calls "print" on the Receiver, but an external
+ * invoker just know he can call "execute" 
+ */
+class HelloCommand implements Command
+{
+
+    protected $output;
+
+    /**
+     * Each concrete command is builded with different receivers.
+     * Can be one, many, none or even other Command in parameters 
+     */
+    public function __construct(Receiver $console)
+    {
+        $this->output = $console;
+    }
+
+    public function execute()
+    {
+        // sometimes, there is no receiver and this is the command which 
+        // does all the work
+        $this->output->write('Hello World');
+    }
+
+}