Warning: Unknown: open(/tmp/sess, O_RDWR) failed: Permission denied (13) in Unknown on line 0

For a cpanel server, this error can exist.

# cd /tmp

particular session file is owned by the user/group of nobody/nobody.

-rw------- 1 nobody nobody 48 Apr 20 04:29 sess_v6bo3v7ta0pirob4lrf8dmrqd2

the rest of the sessions look like the following.

-rw------- 1 domainname domainname 48 Apr 21 06:08 sess_cdc929c2bf695826d3b1e34fe1e65866

1. change the owner of the files to the right user/group or delete the ones that are wrong so that they can be recreated.

The reason they are getting created this way would be because the PHP instance on that site is running as an apache module instead of CGI or fCGI. so the scripts are being run as the apache webserver user a.k.a. nobody/nobody.