Currently (July 2013) 1 &1 uses PHP version 4.4.9 as default PHP version on its shared hosted servers as can be tested with php -v. Many content management systems like Joomla or Drupal need more advanced PHP versions. In order for those platforms to run/install, add this to your .htaccess:

AddType x-mapp-php6 .php
AddHandler x-mapp-php6 .php

These directives tell the webserver to use the newer PHP version – php6 -v yields 5.4.17 – for processing php files.