PHP stands for Hypertext Preprocessor (no, the acronym doesn't follow the name). It's an open source, server-side, scripting language used for the development of web applications. By scripting language, we mean a program that is script-based (lines of code) written for the automation of tasks.
What does open source mean? Think of a car manufacturer making the secret to its design models and technology innovations available to anyone interested. These design and technology details can be redistributed, modified, and adopted without the fear of any legal repercussions. The world today might have developed an amazing supercar!
Web pages can be designed using HTML. With HTML, code execution is done on the user's browser (client-side). On the other hand, with PHP server-side scripting language, it's executed on the server before it gets to the web browser of the user.
PHP can be embedded in HTML, and it's well suited for web development and the creation of dynamic web pages for web applications, e-commerce applications, and database applications. It's considered a friendly language with abilities to easily connect with MySQL, Oracle, and other databases