Added base signup page

public/signup/.htaccess

@@ -0,0 +1 @@
+../.htaccess

public/signup/index.php

@@ -0,0 +1,20 @@
+<?php require_once '/src/base.php' ?>
+<?php
+
+# process URI
+$path = trim($_SERVER['REDIRECT_URL'], '/');
+$args = explode('/', $path);
+
+# no args is very unlikely, but still
+if (count($args) === 0) {
+	die('Unexpected error');
+}
+
+# get the signup method from URL
+$signup_method = $args[0];
+if (!in_array($signup_method, ['mastodon'])) {
+	die('The given arguments are not correct');
+}
+
+# show arguments
+var_dump($signup_method);