Check your parameters.ini file in app/config , and confirm, if db configuration is done correctly. Use database driver as pdo_mysql .
If dosen't work install pdo & pdo_mysql .
pecl install pdo
pecl install pdo_mysql
and add entries in php.ini file
extension=pdo.so
extension=pdo_mysql.so
You can confirm if driver is installed or activated by php_info()