MySQL is a database management system that is used by PHP to store and retrieve all your blog information. Think of it this way. If your database is a filing cabinet that PHP uses to organize and store all the important data from your website (posts, pages, images, etc), then MySQL is the company that created this special type of filing cabinet.
MySQL is an open-source relational database management system. It runs as a server and allows multiple users to manage and create numerous databases. It is a central component in the LAMP stack of open-source web application software that is used to create websites. LAMP stands for Linux, Apache, MySQL, and PHP. Most PHP installations use the LAMP stack because it is open source and works seamlessly with PHP.