Nginx/FPM/PHP all php files say 'File not found.'

Put "include fastcgi_params;" before all "fastcgi_param *" lines, "include fastcgi_params;" overrides all you "fastcgi_param *" lines (see nginx debug log):

location ~ \.php$ {
    root                    /opt/nginx/html;
    fastcgi_pass            unix:/tmp/fpm.sock;
    fastcgi_index           index.php;
    include                 fastcgi_params;
    fastcgi_param           SCRIPT_FILENAME /opt/nginx/html$fastcgi_script_name;
}

I had the same issue.

What I did to solve this was to check the user running nginx, php-fpm and check their permissions for accessing the folder where the root is. It is be default 'www-data'.

but you can find out by using the ps aux | grep php-fpm and ps aux | grep nginx commands.

You have to make sure the folder is accessible to the user running these processes.

I had the same issue, and for me it was a misconfigure/non-existing "root" directive in the nginx server config

I use such configuration, wish it could help you. It works in OS X. As for me, @Xosofox 's answer worked. I mistyped the version of nginx 1.6.2 as 1.2.6, so that the root became a non-existing directory.

location ~ \.php$ {
    fastcgi_intercept_errors on;
    root           /usr/local/Cellar/nginx/1.6.2/html;
    fastcgi_pass   127.0.0.1:9000;
    fastcgi_index  index.php;
    include        fastcgi_params;
    fastcgi_param  SCRIPT_FILENAME /usr/local/Cellar/nginx/1.6.2/html$fastcgi_script_name;
}