PHP Warning Permission denied (13) on session_start() [duplicate]
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. Details are here.
Do a phpinfo(), and look for session.save_path. The directory there needs to have the correct permissions for the user and/or group that your webserver runs as.