HTML stands for Hyper Text Markup Language. With HTML, you can create your own Website. It describes the structure of Web pages using markup.
PHP stands for "PHP: Hypertext Preprocessor". It is a widely-used Open Source general-purpose scripting language especially suited for web development. It can be embedded into HTML. The main goal of the language is to allow web developers to write dynamically generated web pages quickly, but you can do much more with PHP.
Many dynamic websites require a back end database. The database can contain information that the web pages display to the user, or the purpose of the database might be to store information provided by the user. In some applications, the database both provides available information and stores new information.
MySQL is the most popular database for use in websites. It is particularly popular for use with websites that are written in PHP as PHP and MySQL work well together.