PHP session directory full?
sudo find /var/lib/php/session -type f -delete will work better than sudo rm /var/lib/php/session/* because expansion of * is hardly possible for big lists of files.
It sounds like your application is creating sessions but never destroying them. This could be poor programming and a mix of bad environment settings.
If you want to remove those session files you need to use sudo as they're likely not owned by your user.
sudo rm /var/lib/php/session/*
I would not remove the entire session directory.
Another thing is to adjust the PHP Session timeout to a smaller interval.