Can't combine alias with fastcgi_cache in Nginx - php

Question

This works:
location ~ ^/special/(.+\.php)$ {
alias /var/special/$1;
try_files "" =404;
include fastcgi_params;
fastcgi_pass 127.0.0.1:9000; # php-fpm socket
}
But this doesn't:
location ~ ^/special/(.+\.php)$ {
alias /var/special/$1;
try_files "" =404;
include fastcgi_params;
fastcgi_pass 127.0.0.1:9000; # php-fpm socket
fastcgi_cache mycache;
}
If I try to go to the URL "/special/index.php" I get a "File not found." text in the browser, which I assume comes from php-fpm or PHP. And I get this error in the Nginx log:
FastCGI sent in stderr: "Primary script unknown", client: 202.179.27.65, server: myserver.org, request: "GET /special/index.php HTTP/1.1", host: "myserver.org"
Any idea why adding fastcgi_cache breaks this?
Note that fastcgi_cache works fine when I use a location that doesn't use an alias.

Answer 1

After a few days (!) of fiddling with it, this variation seems to work:
location ~ ^/special(/.+\.php)$ {
root /var/special;
try_files "" =404;
include fastcgi_params;
fastcgi_pass 127.0.0.1:9000; # php-fpm socket
fastcgi_cache mycache;
fastcgi_param SCRIPT_FILENAME $document_root$1;
fastcgi_param SCRIPT_NAME $1;
}
What seemed to make the difference is 1) using "root", which seems to be needed by the fastcgi_cache, and 2) explicitly setting SCRIPT_FILENAME and SCRIPT_NAME, because otherwise the "root" wouldn't work (even without fastcgi_cache).