CRON执行的PHP脚本是否受到任何限制？

I have a CRON task set up and it is being called at the correct intervals. The PHP script it is calling will only execute commands written directly into that script, like mail();. But when I try and call another class (with require_once())like class.PHPMailer.php from that script that will not work. I've tried absolute and relative path.

Is a CRON executed PHP script restricted in any way?