I recently upgraded my Ubuntu 10.04 dev server to 14.04. It is actually a fresh install. PHP version was 5.4.15, and is now PHP 5.5.9. MySQL went from 5.1.67 to 5.5.37.
Installing php5-mysqlnd to replace php5-mysql fixed the problem:
sudo apt-get install php5-mysqlnd
It will automatically remove php5-mysql.
I'm guessing this is a bug either in php5-mysql or the underlying libmysqlclient.