session_start warnings

If you get errors like this in your site header:

Warning: session_start() [function.session-start]: open(xxx) failed: Permission denied (13) in xxx/wp-content/plugins/woocommerce/woocommerce.php on line 80

your server is to blame. PHP Sessions may not be set up correctly, or the sessions directory (usually /tmp) is not writable. You need to contact your hosting provider to resolve this.