Uncaught exception: 'The mysql driver is not currently installed'
More information

In /var/www/piwik/libs/Zend/Db/Adapter/Pdo/Abstract.php on line 104
Backtrace:

#0 /var/www/piwik/libs/Zend/Db/Adapter/Abstract.php(228): Zend_Db_Adapter_Pdo_Abstract->_connect()
#1 /var/www/piwik/modules/Piwik.php(809): Zend_Db_Adapter_Abstract->getConnection()
#2 /var/www/piwik/modules/FrontController.php(315): Piwik::createDatabaseObject()
#3 /var/www/piwik/index.php(51): Piwik_FrontController->init()
#4 {main}

You can get help from http://piwik.org (give us the full error message + your PHP and Mysql version)

And this exception raised another exception "No entry is registered for key 'logger_exception'"