Caching objects served from PHP

[jwilson]

I have a server set up with nginx and PHP-FPM, running a framework that
essentially sends all requests for content to PHP, which then serves it
based on ACL rules.  This includes images and other static content.

What I am trying to figure out is how I can get nginx to cache such
objects to avoid the call to PHP for identical data, ie. a static image
that hasn't changed.  

Since all these calls are essentially dynamic, and often the image
objects won't have easily identifiable extensions, I'm not sure how I
might go about caching them.

I could put Varnish in front of nginx and have it do the caching but
that would not allow caching of HTTPS traffic.

Suggestions welcome!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,213634,213634#msg-213634