This might be new error for you but this appears when you try to install your wordpress in your local machine or web server.
This error suddenly appears on my ubuntu machine after fresh install of apache, mysql, and php.
Your PHP installation appears to be missing the MySQL extension.
What to do?
1. Check your php version by:
2. Update the system and install package
sudo apt-get update
sudo apt-get install php7.0-mysql
3. Restart the Web Server
systemctl restart apache2.service