Well, deleting the contents of the directory will not really be good enough. The problem is that the directory itself is probably several MB big. Do "ls -l /var/lib/php5" to see how much space the directory listing is using. You don't want to leave all that space allocated.

mv /var/lib/php5/session /var/lib/php5/session.old
mkdir /var/lib/php5/session
chmod 777 /var/lib/php5/session
chmod +t /var/lib/php5/session
rm -r /var/lib/php5/session.old