Undefined index: HTTP_HOST

Helped out a colleague with this issue. It can occur if you are trying to browse to a local site on your machine via https instead of http (among many other reasons). In this particular case it was due to https-everywhere extension in Firefox. So just make sure you are really going
to a http version of the site.

Another related error you might see is:

SSL received a record that exceeded the maximum permissible length. 
(Error code: ssl_error_rx_record_too_long)

Which immediately tells you your browser is trying to perform a https request.