The location of the log file is configurable in the php.ini so we cannot easily tell you where your log file is as people change it, and it can vary based on linux distro. The log should (IMO) be in /var/log/
If you look through your php.ini file (should be located off /etc/ or /etc/php/ somewhere) and look for the "error_log" directive it should be followed by the path to the php error log.
Once you find the log if you post the error here we may be able to assist you further. :)
