$_SERVER['DOCUMENT_ROOT']在通过cron运行的php脚本中不能工作

I use $_SERVER['DOCUMENT_ROOT']."/lib/sft_required.php"; to include the 'sft_required' file in a PHP script. When I run this file using browser, it works fine but when I run this as a cron job job, it does not work. It seems that the file is not included when we run the script through cron.I use $_SERVER['DOCUMENT_ROOT']."/lib/sft_requi