Here's something you might want to add to any function's documentation that has to do with writing to files - like fopen(), session_start(), etc.

Apache's default httpd.conf currently ships with a section that looks like this:

User nobody
Group "#-1"

There is a section above that with a somewhat confusing instruction to use "Group nogroup" if you run SCO 3. It doesn't mention Linux, but does go into details of why you shouldn't use #-1 if your kernel can't use GIDs above 60,000. Let's assume some or most people tuning httpd.conf don't know whether that's true about Linux. :)

Anyway, if your httpd.conf looks like that under Linux (2.4 for sure, dunno about 2.2), PHP will not be able to open files. It'll complain about an invalid argument. That applies to session_start() and fopen() - I've tried both.

The solution is to use "Group nogroup". Once I changed my httpd.conf to use that instead of #-1, I was suddenly able to write files again.