how to fix 'Mysql has gone away'

just ensure you have swap file and add to /etc/mysql/my.cnf :

max_allowed_packet=32M
wait_timeout=172800

How to solve PDOException “could not find driver” in Ubuntu

sudo apt-get install php5-mysql

It can answer that will be conflicts with some packages, and need to remove it, but suddenly that conflict was solved by it own. After "php5-fpm -m" command you should get pdo_mysql.