The chown command has the following syntax:

chown username:groupname directory

So in your example command it is your primary group name you need to put
To determine your current user name issue the command as below on a terminal

whoami

This will return your current user name. Then issue this command to determine your group memberships

groups username

Assuming that your username is "alex", run:

groups alex

You will get something like:

alex : alex staff adm cdrom lpadmin admin

The first one after the : is your primary group. In this example it is "alex".

The command will be:

sudo chown -R root:alex /var/lib/php/session