Warning: session_write_close() [function.session-write-close]: open(/tmp/sess_1234567890, O_RDWR) failed: Permission denied (13) in /script

If you get an error similar to:

Warning: session_write_close() [function.session-write-close]:  open(/tmp/sess_1234567890, O_RDWR) failed: Permission denied (13) in /script

it would imply that your /tmp folder is not writeable by php.  The /tmp folder is a globally shared writing location and the permission should be set to 1777. This allows everyone write access to it, but will prevent cross-user account writing (so users cannot write other's files).

To fix the above error, type:

chmod 1777 /tmp