Cache is on, but var/cache is empty?

Could also be you are using a different cache storage like memcached, Redis

If you have a var dir or even a var/cache dir, but Magento can't write to it, Magento will use sys_get_temp_dir() to determine a good place to put the caching files.

Often this is /tmp/ and results in /tmp/magento/var/cache (Thanks to fschmengler)

I ran into this problem when magento writes it cache file to /tmp/magento/var/cache folder.

This is weird issue, I had solved this SELinux issue by following command.

$ sudo restorecon -R /var/www/html
$ sudo setsebool -P httpd_unified 1