This is a possible issue when the path to save php_session is not correctly set :
The directory for storing session does not exists or php do not have sufficient rights to write to it.
To define the php_session directory simply add the following line to the php.ini :
And give write rights to the http server.
usually, the http server run as user daemon in group daemon. If it is the case, the following commands will make it :
chown -R :daemon /tmp/php_session
chmod -R g+wr /tmp/php_session
restart http server.