Posting in the Magento forums has been disabled pending the implementation of a new and improved forum solution which should better serve the community.
For new questions please post at magento.stackexchange.com, the community-run support site for the Magento community. We will be providing updates on the new forum solution soon. For questions or concerns please email firstname.lastname@example.org.
If Magento cannot write to var/cache/ and var/tmp/, it then proceeds to write to the system /tmp folder (See image attachments on post in above link showing this in action). Once config is cached in the system tmp folder, you will never get the site to work until you fix things.