PHP is the most popular server side scripting language widely used in web development to create dynamic web pages. PHP can be deployed in 99% of web servers available and also compatible with almost all the Operating Systems and Platforms free of charge. More than 50 million websites are coded using PHP and related frameworks.
MySQL is a program that can store large amounts of information in an organized format that is easily accessible from scripting languages like PHP. The whole idea of a database-driven Web site is to allow the content of the site to reside in a database, and for that content to be dynamically pulled from the database to create Web pages featuring it for people using a regular Web browser to view. So on one end of the system you have a visitor to your site using a Web browser, loading your site and expecting to view a standard HTML Web page. On the other end you have the content of your site sitting in one or more tables in a MySQL database that only understands how to respond to SQL commands.
We do Content Managing Systems (CMS) using PHP and MySQL. The PHP scripting language is the go-between that speaks both languages. Using PHP, we write the presentation aspects of your site as templates in regular HTML. Where the content belongs in those templates, we use some PHP code to connect to the MySQL database and using SQL queries to retrieve and display some content in its place.
We develop Web Applications too using PHP/MySQL. Now a days we use PHP/MySQL to develop Android Apps as well.