I finally make it right myself.

server {    listen       80;    server_name  example.com;    root   /home/wwwroot/example.com;    index index.php  index.html index.htm;    location / {    root /home/wwwroot/example.com;            index  index.php index.html index.htm;        if (!-e $request_filename) {                rewrite ^/(.*)$ /index.php/$1 last;                break;        }    }    location ~ \.php($|/) {        fastcgi_pass  unix:/tmp/php-cgi.sock;        fastcgi_index  index.php;        fastcgi_split_path_info ^(.+\.php)(.*)$;        fastcgi_param   PATH_INFO $fastcgi_path_info;        fastcgi_param  SCRIPT_FILENAME  $document_root$fastcgi_script_name;        include        fastcgi_params;    }    location ~ /\.ht {            deny  all;    }}

please add the following line to Nginx configuration file /etc/nginx/nginx.conf

http {    ...    fastcgi_buffers 8 16k;    fastcgi_buffer_size 32k;    ...}

reference

I was also getting this error on Codeigniter + nginx but i have solved it by changing my code.The problem is with the session. In the Session i was saving the stdClass object. When i change the value or retrieve the value from session it gives me 502 bad gateway. So i change the session value to Associative Array and then my problem is solved. I think session storage value get exceed this is why the server give the error 502 bad gateway.