added Visitor pattern

Visitor/Visitor.php

@@ -0,0 +1,97 @@
+<?php
+
+namespace DesignPatterns;
+
+/**
+ * Visitor Pattern
+ *
+ * Purpose:
+ * The Visitor Pattern lets you outsource operations on objects to other objects. The main reason to do this is to keep
+ * a seperation of concerns. But classes have to define an interface to allow visitors (the "accept" method in the example below).
+ * 
+ */
+interface RoleVisitor {
+    public function visit(Role $role);
+}
+
+interface Role {
+    public function getName();
+}
+
+class RolePrintVisitor implements RoleVisitor {
+
+    /**
+     * @param Role $role
+     */
+    public function visit(Role $role) {
+        echo "Role: " . $role->getName();
+    }
+}
+
+class User implements Role {
+
+    /**
+     * @var string
+     */
+    protected $name;
+
+    /**
+     * @param string $name
+     */
+    public function __construct($name) {
+        $this->name = (string) $name;
+    }
+
+    /**
+     * @param RoleVisitor $visitor
+     * @return void
+     */
+    public function accept(RoleVisitor $visitor) {
+        $visitor->visit($this);
+    }
+
+    /**
+     * @return string
+     */
+    public function getName() {
+        return "User " . $this->name;
+    }
+}
+
+class Group implements Role {
+
+    /**
+     * @var string
+     */
+    protected $name;
+
+    /**
+     * @param string $name
+     */
+    public function __construct($name) {
+        $this->name = (string) $name;
+    }
+
+    /**
+     * @param RoleVisitor $visitor
+     * @return void
+     */
+    public function accept(RoleVisitor $visitor) {
+        $visitor->visit($this);
+    }
+
+    /**
+     * @return string
+     */
+    public function getName() {
+        return "Group: " . $this->name;
+    }
+}
+
+$user = new User("Dominik");
+$group = new Group("Administrators");
+
+$printer = new RolePrintVisitor();
+$printer->visit($user);
+$printer->visit($group);
+