The '/var/lib/php/session/' directory should be writable by default. On occasion, certain software will refuse to see this path as writable. In that instance what you will need to do is create a special file called vhost.conf that will tell that domain to use another location you specify:

Create the new directory to house your sessions inside of your domain directory:

mkdir /var/www/vhosts/mt-example.com/tmp 

Apply the appropriate permissions and file ownership:

chown username:psaserv /var/www/vhosts/mt-example.com/tmp && chmod 777 /var/www/vhosts/mt-example.com/tmp 

Create the vhost.conf file in the following directory /var/www/vhosts/mt-example.com/conf using the following command:

 vi /var/www/vhosts/mt-example.com/conf/vhost.conf

  php_value session.save_path /var/www/vhosts/mt-example.com/tmp 

Save the file and run this command to force Plesk to see the changes:

/usr/local/psa/admin/sbin/websrvmng --reconfigure-vhost --vhost-name=mt-example.com 

Restart Apache to load the new changes: 

service httpd graceful

Your software should now use the new path for sessions data and your CMS installation should proceed.

Revisions:

07-20-2009: Minor Fixes