running crontab via web gives error that nobody does not have access to run crontab

Question

I'm having such a hard time getting this to work. I have looked all over the place. Here's the situation.

I'm hosting in a shared environment. I am dynamically creating a crontab file and trying to execute is using exec command. Everything runs fine when I do this via telnet but when I run my php file that runs this exec command, I get an error saying that

"You (nobody) are not allowed to use this program (/usr/bin/crontab)"

How do I make sure I (nobody) have access to run this crontab command.