Another post recommended changing the permissions on the PHPost directories to 777. This allows everyone to read, write, and execute all of the files. I may be wrong, but that is a security concern.
Either way, the Readme file for PHPost recommends the following:
If you are running PHP in safe mode, the directory will have to be chgroup'd to the same group that your web server runs as (usually 'nobody'). You will also want to change the permissions to "drwxrwx---" (that's 770).
changing the group to nobody didn't seem to work for me, so I changed the owner to www (AFAIK the same owner as Apache.). Then I changed the Permissions to 770 (only owner and group can read, write.) and everything works fine. Great little program.