You'll need to post the raw contents of the file specified (/xxxx/xxx/xxxx/xxxxx/cache_builders/custom/portal_box_52.php) in order to have us troubleshoot why its not working.
It sounds like you entered <?php in the custom island editor; items in the editor are already an open PHP file, so no <?php ?> should be needed (when using the editor). Note that when opening the file outside of the editor you'll see the standard <?php ?> at the beginning/end of the file.
