Linux Drush install error: Undefined class constant 'MYSQL_ATTR_SSL_CA' in phar:///usr/local/bin/drush/lib/Drush/Sql/Sqlmysql.php, line 56
In my case, not all of the PHP packages made it though the Ubuntu upgrade to 16.04 (which now runs PHP 7 by default).  I needed to add the missing one, and then Drush started working again:

sudo apt install php-mysql


Or for php5 users

sudo apt install php5-mysql 


on very recent Ubuntu/Debian distributions.

For PHP 7.1:

sudo apt install php7.1-mysql


For PHP 7.2:

sudo apt install php7.2-mysql


You can search for your version using this:

apt-cache search php-mysql