PHP run from cron fails occasionally

This is the strangest thing I've encountered yet. I thought the original 
problem was the "-f" in the command, so I removed     it. Still, every 
couple days the command that runs the PHP script fails to execute. Since 
no log is generated, I don't know what else to do to diagnose this. If I 
run the script from the terminal, it always works, creates the report 
and mails it. I've even looked in syslog for when the cron job runs, and 
it indeed runs to completion with no errors. Suggestions as to how to 
trace this one down most welcome.