I answered this on Twitter but i’m going to answer at slightly more length here, both for posterity and because frankly i think that Dan Decker’s answer was pretty uncalled for and shortsighted. Damien wasn’t asking about Health Check, he was asking about an underlying environment problem that Health Check had turned up, and whether that underlying problem is going to have any effect on an EE installation.
The answer is that session_save_path is a PHP variable that is set in php.ini, and on many shared hosting setups it’s actually set to a location that’s not writable. It usually doesn’t actually matter; EE is capable of doing session management without it unless something goes really wrong. Some addons, however, may NOT work properly without it.
Damien: the issue with the solution your host implemented is that anytime you upgrade EE you’ll have to remember to make those mods to the admin.php file. That MAY be the best solution you can manage. However, assuming that Apache is configured to allow setting PHP overrides in .htaccess (which it may or may not be, on a shared host, but it’s a better than even chance that it will be), you should just be able to add the following two lines to the .htaccess file in your webroot:
php_value session.save_handler files
php_value session.save_path "/path/to/tmp"
The way to tell whether or not you’re allowed to set these values is: set them, and then try to load your site. If you get a “500 Server Misconfiguration Error”, you can’t override these values in .htaccess, and you’re stuck with the admin.php edit. If everything loads fine, you can just delete the lines your host added to admin.php and go about your day!