[Excel]

Hello everybody, I have a problem...
I might be blind or something, but is there a list of commands the cronjobs can't run?

I have a PHP script, which creates 2 files.
If I (or anybody else) calls the php file manually, the 2 files are created.
If the cronjob calls the script however, the calculations are done,
and the database is updated, only the 2 file are not created.
(The directory permission is 777)

So, can a cronjob not run fopen, or did I make some mistake?