[tlug] Appropriate permissions for cron job PHP script

[dave@example.com]

TLUG,

I have a PHP script on one of my sites that does a regular backup.  
It's activated by a cron job.

I realized this morning that it if someone knew the exact URL, they  
could activate it and if they hammered it enough, I suppose they could  
cause some slow down on the site.

I was wondering what the appropriate permission setting would be to  
make it so that it can only be activated by the local system, not by  
visitors to the site.

I think that should be possible. But to say the least, my  
experimentation only seems to make it not work for anyone.

Is what I intend to do possible?

Any advice much appreciated.

--
Dave M G