The MySQL PHP extension can helps to access the MySQL database on your PHP application, by default this extension is not installed in your instance, this post walk you through how to install extension on your Ubuntu and CentOS Operating Systems.

On Ubuntu:

apt-get update
apt-get install php-mysql

On CentOS:

yum update
yum install php-mysql

Restart the webserver,

If you are using Ubuntu operating system use the command below,
service apache2 restart
service nginx restart

If you are using CentOS operating system use the command below,\
service httpd restart
service nginx restart