Coloque "include fastcgi_params"; antes de todas as linhas "fastcgi_param *", "include fastcgi_params;" substitui todas as linhas "fastcgi_param *" (consulte o log de depuração nginx):
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;
}