added simple Dependency Injection example

DependenyInjection/DependencyInjection.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace DesignPatterns;
+
+/**
+ * Dependency Injection
+ *
+ * Purpose:
+ * to implement a loosely coupled architecture in order to get better testable, maintainable and extandable code
+ * 
+ * Examples:
+ * - the Doctrine2 ORM uses dependency injection e.g. for Configuration that is injected into a Connection object. for
+ *   testing purposes, one can easily create a mock object of the configuration and inject that into the connection
+ *   object
+ * - Symfony and Zend Framework 2 already have containers for DI that create objects via a configuration array and
+ *   inject them where needed
+ *
+ */
+class Configuration
+{
+    protected $_host;
+
+    /**
+     * @param string $host
+     * @return Configuration
+     */
+    public function setHost($host)
+    {
+        $this->_host = $host;
+        return $this; // for a fluent interface
+    }
+
+    /**
+     * @return string
+     */
+    public function getHost()
+    {
+        return $this->_host;
+    }
+}
+
+class Connection
+{
+    protected $_configuration;
+
+    /**
+     * here, Configuration gets injected and Connection will get all that it needs from Configuration
+     * without DI, the configuration would be created directly in Connection, which is not very good
+     * for testing and extending Connection
+     *
+     * @param Configuration $config
+     */
+    public function __construct(Configuration $config)
+    {
+        $this->_configuration = $config;
+    }
+
+    public function connect()
+    {
+        $host = $this->_configuration->getHost();
+        // ...
+    }
+}