set permission to absolute 777

This is a very wrong tip.

Just follow the documentation: http://learn.elgg.org/en/latest/intro/install.html#create-a-data-folder

Seriously? Are you trying to say that the documentation is wrong? Set this permissions on your data path, give me your website URL and I'll show you many interesting things with your server ;)

The important point is "outside document root". Then no one can access the data directory over the Internet directly. The second point is folder permissions. Important is that the webserver can access/read/write the data directory. So, you can either change the owner of the data directory to the account (on the server) used by the webserver process. Then you don't need 777 permissions. The 777 permissions would just be the simplest "solution" if you don't want to (or can't for whatever reason) change the owner and/or group of the data directory accordingly for the webserver being able to access it properly. But most importantly: data directory ALWAYS outside the document root directory tree. The docs as linked above should be detailed enough to understand the pros and cons of the different options regarding permissions of the data directory.