added observer pattern

Observer/Observer.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace DesignPatterns;
+
+/**
+ * Observer pattern
+ *
+ * Purpose:
+ * to implement a publish/subscribe behaviour to an object, whenever a "Subject" object changes it's state, the attached
+ * "Observers" will be notified. It is used to shorten the amount of coupled objects and uses loose coupling instead
+ *
+ * Examples:
+ * - a message queue system is observed to show the progress of a job in a GUI
+ *
+ * PHP already defines two interfaces that can help to implement this pattern: SplObserver and SplSubject
+ *
+ */
+class UserObserver implements \SplObserver
+{
+    public function update(\SplSubject $subject)
+    {
+        echo get_class($subject) . ' has been updated';
+    }
+}
+
+class User implements \SplSubject
+{
+    /**
+     * @var array
+     */
+    protected $_observers = array();
+
+    /**
+     * attach a new observer
+     *
+     * @param \SplObserver $observer
+     * @return void
+     */
+    public function attach(\SplObserver $observer)
+    {
+        $this->_observers[] = $observer;
+    }
+
+    /**
+     * detach an observer
+     *
+     * @param \SplObserver $observer
+     * @return void
+     */
+    public function detach(\SplObserver $observer)
+    {
+        $index = array_search($observer, $this->_observers);
+
+        if (false !== $index) {
+            unset($this->_observers[$index]);
+        }
+    }
+
+    /**
+     * 
+     *
+     * @return void
+     */
+    public function notify()
+    {
+        /** @var SplObserver $observer */
+        foreach ($this->_observers as $observer) {
+            $observer->update($this);
+        }
+    }
+}