I'm using Joomfish component then it give error "500 not valid database" For my Component. When I changed my database MySQL to mysqli then it's working fine. Why?
Unless you are running PHP 5.3, the mysqli extension is not installed by default. You probably haven't installed it, or it isn't configures properly.
Please check the phpinfo() function to see if it is installed and read about setting up mysqli to see how to install and/or configure it.