Hard to speculate what 'simply does not run' means. I cooked up a quick couple pages on debian which work ok as a test.
p1.php:
<?php
require_once('p2.php')
?>
p2.php:
<?php
phpinfo()
?>
Drop both of them in your web document root and open http://hostname/p1.php
and you should see it execute the stuff in p2.php
