The nginx 1.6.1-2 release on Debian Backports made a change to the default fastcgi configuration file with an almost casual reference in the release notes:
+ Sync with upstream and remove `SCRIPT_FILENAME` parameter.
This change might break fastcgi sites. (Closes: #718639)
Talk about an understatement – that change broke every single PHP based site I manage (including this blog). Fixing the errors from this change …