The error messages you are getting seem to be all the same issue. It appears that the last error you are getting is the one that lets you know why the others are occurring:
Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/php_sessions) in Unknown on line 0
This means that in your PHP setup the session save path is either not specified correctly or you do not have permissions to read and write to this location. Based on the first message it would seem that the location does not exist. If you admin the server you should check this setting in the ini file to ensure that it is set to the correct location and that the location exists on the server.
If you are not the admin you should contact your host to ask them to look into this for you. Anyone who is administering the php part of the server should know what these errors are and be able to help you get past them.