MySQL is a freely available open source Relational Database Management System (RDBMS) that uses Structured Query Language (SQL).
A MySQL database is what WHMCS uses to store all of the data your WHMCS system controls.
MySQL is the only supported database engine by WHMCS at this time.
PHP Data Objects, or PDO for short, is a database access layer that provides a uniform method of access to multiple databases.
In WHMCS 6.0 and later, the PHP environment in which you run WHMCS must support PDO. This requires the loading of two PHP extensions in your PHP environment:
For more information, please refer to http://php.net/manual/en/ref.pdo-mysql.php
Strict SQL Mode
MySQL Strict Mode controls how MySQL handles invalid or missing values in data-change statements such as INSERT or UPDATE.
For more information on SQL Modes, and how to set the SQL Mode for your MySQL environment, please refer to MySQL's official documentation available at the url below.