Class 'PDO' not found
From Joomla! Documentation
In Joomla 3.4 a new, based on the PDO, database driver for MySQL, was added (rapidly becoming the new de facto driver for MySQL database connections). However an incomplete check for the existence of the PDO driver was implemented for some users who didn't have the driver installed with PHP.
Fatal error: Class 'PDO' not found /libraries/joomla/database/[drive]/pdomysql.php on line 102
What is the cause
PHP doesn't ship in some cases with the PDO driver (it is present in the majority of instances) and there was a missing check for the existence of the PDO Driver.
How to fix
Apply the patch https://github.com/joomla/joomla-cms/pull/6175. This will be included in Joomla 3.4.1