<?php
$routes = [
  'doppelklinge' => 'doppelklinge',
  'vermessung' => 'vermessung',
  'default' => 'default'
];

ob_start();
if(array_key_exists($_page, $routes))
  require_once("./pages/{$routes[$_page]}.page.php");
else
  require_once("./pages/{$routes['default']}.page.php");

$_content = ob_get_contents();
ob_end_clean();