این مورد رو از سرچ ارور شما توی گوگل پیدا کردم که بهتره بررسی کنید

You don't appear to have write permission to the /tmp directory on your server. This is a bit weird, but you can work around it. Before the call to session_start() put in a call to session_save_path()and give it the name of a directory writable by the server.