Php, session.save_path

Php, session.save_path

I wanted to ask about session.save_path from php.ini file
(http://php.net/session.save-path), My question is where is the session is
saved if i am not setting any path(;session.save_path =), Is it on my PC
memory?, Is it on files that located outside php directory?,So far i see
that with session.save_path set, I can view the files, And when
session.save_path is unset(http://php.net/session.save-path), I don't know
where the files if there is any, If anyone can help me i will be very
thankful, Thank you all and have a nice day.