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

Command/Receiver.php

@@ -0,0 +1,20 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\Command;
+
+/**
+ * Receiver is specific service with its own contract and can be only concrete
+ */
+class Receiver
+{
+
+    public function write($str)
+    {
+        echo $str;
+    }
+
+}