--- a/lib/openid-php/Auth/OpenID/Extension.php
+++ b/lib/openid-php/Auth/OpenID/Extension.php
@@ -1,1 +1,62 @@
+<?php
 
+/**
+ * An interface for OpenID extensions.
+ *
+ * @package OpenID
+ */
+
+/**
+ * Require the Message implementation.
+ */
+require_once 'Auth/OpenID/Message.php';
+
+/**
+ * A base class for accessing extension request and response data for
+ * the OpenID 2 protocol.
+ *
+ * @package OpenID
+ */
+class Auth_OpenID_Extension {
+    /**
+     * ns_uri: The namespace to which to add the arguments for this
+     * extension
+     */
+    var $ns_uri = null;
+    var $ns_alias = null;
+
+    /**
+     * Get the string arguments that should be added to an OpenID
+     * message for this extension.
+     */
+    function getExtensionArgs()
+    {
+        return null;
+    }
+
+    /**
+     * Add the arguments from this extension to the provided message.
+     *
+     * Returns the message with the extension arguments added.
+     */
+    function toMessage($message)
+    {
+        $implicit = $message->isOpenID1();
+        $added = $message->namespaces->addAlias($this->ns_uri,
+                                                $this->ns_alias,
+                                                $implicit);
+
+        if ($added === null) {
+            if ($message->namespaces->getAlias($this->ns_uri) !=
+                $this->ns_alias) {
+                return null;
+            }
+        }
+
+        $message->updateArgs($this->ns_uri,
+                             $this->getExtensionArgs());
+        return $message;
+    }
+}
+
+