If anything goes wrong with this connection, PDO is going to output a fatal error that includes the database username and password unless the exception is caught.

http://us2.php.net/manual/en/pdo.connections.php
"If your application does not catch the exception thrown from the PDO constructor, the default action taken by the zend engine is to terminate the script and display a back trace. This back trace will likely reveal the full database connection details, including the username and password. It is your responsibility to catch this exception, either explicitly (via a catch statement) or implicitly via set_exception_handler(). "