when install joomla maybe you find some things about the problem "Session save path  /tmp, Unwriteable".this i not the joomla problem,you need to edit the php.ini file.

My advice is to check the upload_tmp_dir and session.save_path setting in your php.ini file. Look at the directory they point to. If they are not set, then fill directory paths and make sure that they exists on

Of course, after that, you have to restart Apache for changes to take effect.